Propane Dew

8/2/2019 Propane Dew

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May 2008


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Disclaimer

This publication was prepared for the Canadian Association of Petroleum Producers, the GasProcessing Association Canada, the Alberta Department of Energy, the Alberta EnergyResources and Conservation Board, Small Explorers and Producers Association of Canada andNatural Resources Canada by CETAC-West. While it is believed that the information containedherein is reliable under the conditions and subject to the limitations set out, CETAC-West and the

funding organizations do not guarantee its accuracy. The use of this report or any informationcontained will be at the users sole risk, regardless of any fault or negligence of CETAC-West orthe sponsors.

Acknowledgements

This Fuel Gas Efficiency Best Management Practice Series was developed by CETAC WESTwith contributions from:

Accurata Inc.

Clearstone Engineering Ltd.

RCL Environmental

REM Technology Inc. Sensor Environmental Services Ltd.

Sirius Products Inc.

Sulphur Experts Inc.

Amine Experts Inc.

Tartan Engineering

CETAC-WEST is a private sector, not-for-profit corporation with a mandate to encourageadvancements in environmental and economic performance in Western Canada. The corporationhas formed linkages between technology producers, industry experts, and industry associates tofacilitate this process. Since 2000, CETAC-WEST has sponsored a highly successful eco-efficiency program aimed at reducing energy consumption in the Upstream Oil and Gas Industry.

Head Office# 420, 715 - 5th Ave SWCalgary, AlbertaCanada T2P2X6Tel: (403) 777-9595Fax: (403) [email protected]


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Table of Contents

1. Applicability and Objectives .......................................... 1

2. Fundamentals of Refrigeration...................................... 22.1 Refrigeration Circuit2.2 Types of Refrigerant2.3 Energy Consumption Targets

3. Inspection, Monitoring and Record Keeping................ 4

4. Efficiency Assessment and Adjustments..................... 74.1 Compression4.2 Refrigerant Condenser4.3 Economizer4.4 Sub-Cooler4.5 Chiller and Low Temperature Separator4.6 Refrigerant Circuit Piping

5. Facility Engineering Input .............................................175.1 Water for Propane Condensing

5.2 Refrigeration Lean Oil Absorption5.3 Cascaded Refrigeration

6. Appendices.....................................................................20Appendix A Propane P-H DiagramAppendix B Taking Advantage of ClimateAppendix C ChillerLTS CoordinationAppendix D Refrigeration and Other Processes

Appendix E Variable Flow in CompressorAppendix F Condenser PerformanceAppendix G References


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Figures

Figure 2.1 Simple Refrigeration CircuitFigure 2.2 Complex Refrigeration CircuitFigure 3.1 Flow Plan for Reducing Energy in a Refrigeration UnitFigure A.1 Simple Refrigeration CircuitFigure A.2 Pressure-Enthalpy (P-H) Diagram for PropaneFigure A.3 Propane P-H Diagram Simple Refrigeration CircuitFigure A.4 Complex Refrigeration CircuitFigure A.5 Propane P-H Diagram EconomizerFigure A.6 Propane P-H Diagram SubcoolerFigure B.1 Compressor bhp as Function of Discharge PressureFigure C.1 Refrigeration Unit Feed Phase MapFigure C.2 NGL Recovery by LTS Choke ValveFigure E.1 Performance Curves Centrifugal Compressor

Figure E.2 Performance Curves Centrifugal CompressorFigure F.1 Propane Temperature-Pressure Equilibrium

Tables

Table 2.1 Efficiency of Generating RefrigerationTable 3.1 Check Sheet for Compressor OperationTable 3.2 Check Sheet for Condenser OperationTable 3.3 Check Sheet for Economizer OperationTable 3.4 Check Sheet for Subcooler OperationTable 3.5 Check Sheet for Chiller/LTS OperationTable 3.6 Check Sheet for Refrigeration Circuit PipingTable E.1 Compressor Comparison Chart


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EFFICIENT USE OF FUEL GASIN THE UPSTREAM OIL AND GAS INDUSTRY

MODULE 13 of 17: Refrigeration

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Efficient Use of Fuel Gas in Refrigeration Units Rev Date 27/05/2008Module 13 of 17 Page 1 of 52

1. Applicability and Objectives

This module focuses on refrigeration units from the perspective of both theprocess and the refrigerant sides. They are intimately linked and need to beconsidered in tandem in order to achieve the best results.

While the overwhelming majority of refrigeration units operated in the upstreamoil and gas industry (UOG) use propane as the refrigerant, the conceptsdiscussed within this document are applicable to all refrigerants to varyingdegrees. Nevertheless, the examples, unless specifically stated, refer topropane refrigeration.

In this module the terms fuel use and energy use are used interchangeably.Refrigeration units employ engines, turbines and/or electric motors to drive themajor equipment. For those plants where compression is motor-driven theimpact upon unit fuel consumption is obviously much reduced, but the savings

still exist, in the form of reduced electrical power consumption. The energymanagement concepts that are outlined in this document are applicable allcases.

The objectives of this module are to:

Describe the fundamental operation of a refrigeration unit in terms of thethermodynamics involved. This description is for the overall unit and foreach piece of component equipment.

The target audience are the operators of the facilities, although input from

corporate and technical functions elsewhere within the company will benecessary and of vital importance.

Investigate energy management opportunities for the overall process andfor each major piece of equipment both on the refrigerant side and on theprocess gas side.

Extend the study to other processes that are affected by the refrigerationunit and vice versa.


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2. Fundamentals of Refrigeration

Refrigeration is a complex process, involving a long list of operations typicallyfound in upstream oil and gas facilities compression, aerial cooling, heatexchange, instrumentation and controls. Overlaying those operations are the

impacts of thermodynamic cycles and vapourliquid equilibria.

2.1 Refrigerant Circuit

The most basic refrigeration circuit consists of a compressor, condenser,expansion valve and a chiller/evaporator. Typically, in sour gas plants, ethyleneglycol (EG) will be injected into the chiller and upstream process gas exchangers,such as the gas-gas exchanger, in order to prevent water freezing. The EGregeneration equipment is typically located in the refrigeration unit. Energy

management of the EG loop is discussed in another module. Figure 2.1 is aschematic of a simple (single-stage) refrigeration circuit.

Simple Refrigeration Circuit

Compressor

Receiver

Expansion Valve

Chiller

Condenser

Process Gas

Figure 2.1


More complex units will have an economizer or a propane sub-cooler, or both.The increase in equipment complexity results in a much more energy-efficientprocess. Figure 2.2 is a schematic of a complex (two-stage) refrigeration circuit.


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Analysis of the thermodynamics of the refrigeration process using propane -and how to determine equipment and process efficiency are discussed inAppendix A.

Complex Refrigeration Circuit

Compressor

Receiver

Expansion Valve

Chiller

Condenser

Process Gas

Economizer

Subcooler

Figure 2.2


The design of refrigeration units is dictated by the need to compress andsubsequently condense propane during the hottest days of summer. However,the thermodynamics are much more efficient in cold weather. The climate inWestern Canada is therefore very conducive to the realization of energy savingsthrough the effective operation of the refrigeration unit. Appendix B explores theclimatic effect on compression power requirements.

The refrigeration unit is a utility. However, it should not be consideredindependent of the main process. Significant benefits can be achieved bycombining the thermodynamic properties of the process gas and the capabilitiesof the refrigeration unit chiller. These benefits can be in the form of eitherreduced energy consumption for a constant liquid yield, or increased liquid yield.Appendix C discusses this concept.

Refrigeration units are sometimes operated in conjunction with lean oil absorbersfor NGL recovery. Plants that have deep-cut fractionation of the sales gas, whichrequire extremely cold condensing temperatures, combine two refrigerationsystems: the first uses ethane or ethylene for the main process refrigeration andthe second refrigeration loop uses propane to condense the first refrigerant. SeeAppendix D for more information.


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2.2 Types of Refrigerant

The most common refrigerant used in the upstream oil and gas industry ispropane. Other refrigerants are used occasionally, such as ethane, ethylene,propylene, ammonia and various derivatives of Freon.

The ultimate choice of refrigerant is based, to a certain extent, upon availability,price, and compatibility with the rest of the process. However, the major factorwhen choosing a refrigerant is its thermodynamic properties what chillingtemperature can be achieved and what compression power is required.

It should be pointed out that each refrigerant has its own thermodynamicproperties. While they are essentially identical in form, the shape/slope of eachcurve is somewhat different so that the impact of equipment, such as aneconomizer and a sub-cooler, is different.

2.3 Energy Consumption Targets

Power consumption in a refrigeration unit will be a function of the type ofcompressor, the operating mode (dew point control, NGL recovery) and theambient conditions.

The following Table 2.1 lists the brake power input to the compressor per ton ofrefrigeration in the chiller. 1 (A ton of refrigeration TR - is an old unit ofmeasurement, based upon the amount of heat required to melt one ton of ice in aday. It is equivalent to 12,000 BTU/hr.) The values in the table are those forrelatively small Mycom compressors as such, they are representative of manyof the refrigeration compressors in Western Canada.

The left column assumes a condensing temperature of 48.9C, which isequivalent to an ambient temperature of about 32-35C. The right columnassumes a condensing temperature of 20C, which corresponds to an ambienttemperature of 4-6C. This would be fairly representative of spring and autumnconditions and is typical of wintertime operation where pressure control is used tomaintain a minimum operating pressure.

The chiller has been set at -10C, for dew point control, and at -22.7C, for

increased NGL recovery.

The term TR at Full Load indicates the amount of refrigeration that can beproduced when the compressor is at full load. That number multiplied by thenumber brake kW/TR will allow the calculation of the compressor powerrequirement. Power requirements are for brake power output (in kW). For theamount of input energy required, as well as means to reduce that energyconsumption, are discussed in other module chapters.


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Because of the large number of factors that dictate the amount of refrigerationthat is required and the ability to generate that amount, a precise quantification ofan energy benchmark is impossible in a simple table. Nevertheless, the tabledoes give an indication of the approximate energy intensity that should be set as

an initial target.

Table 2.1Efficiency of Generating Refrigeration

Condensing Temperature, C 48.9 20.0

Reciprocating Compressor

Single Stage

Mycom F12WB

Chiller Temperature, C -10.0 -10.0

TR at Full Load 121.7 186.6Brake kW/TR 1.58 0.75


TR at Full Load 63.2 108.4

Brake kW/TR 2.22 1.13

Screw Compressor

No Economizer

Mycom P200 VS-M







Screw Compressor

With Economizer

Mycom P200 VS-M



Brake kW/TR 1.62 0.82Chiller Temperature, C -22.7 -22.7




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Note from Table 2.1 how the ability to generate refrigeration, both in terms ofquantity and efficiency of generation, decreases as the condensing temperaturerises and/or the chiller temperature is reduced.

Note also how the use of an economizer will improve the quantity of refrigeration

and generally increase the efficiency of that generation. The same screwcompressor model was used in both the no economizer/economizer cases.


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4. Efficiency Assessment and Adjustments

Whether the plant has a simple or complex configuration, energy useoptimization is performed in a similar manner. Figure 3.1 is a flow plan forevaluating a refrigeration unit.

Confirm Measurement

Accuracy

Identify Opportunities/

Calculate Benchmark

Data Analysis/

Reconciliation

Data Collection

Repair, Replace,

Install

Form Refrig'n Unit

Energy Mgmt Team

N

Poor/Missing

Data?

Y

Piping

(Section 3.6)

Chiller/LTS

(Section 3.5)

Subcooler(Section 3.4)

Economizer

(Section 3.3)

Condenser

(Section 3.2)

Compressor

(Section 3.1)Investigate

Opportunities

Set Study

Goals

Discard

Implement

Opportunities

Opportunities

Feasible?

Determine/Confirm

Operating Targets

Evaluate

Opportunities

Y

N

Figure 4.1Flow Plan for Reducing Energy in a Refrigeration Unit

When conducting an energy management study of the refrigeration unit the mainparameters to track are the absolute amount of energy consumed. Absoluteamount of energy consumed should be represented in terms of brake power onthe compressor and refrigeration obtained, for example, tons of refrigeration perbrake compressor power. In thermodynamics, this is often referred to as thecoefficient of performance.

The energy consumed can also be expressed and tracked as a function of theplant processing conditions the amount of power compared with the productquality. For a unit that is designed for dew point control, the function would bepower versus sales gas dew point. For a plant that wants NGL recovery, it wouldbe power versus the NGL production.

Ambient conditions need to be accounted for, in view of the tremendous impactthat condensing temperature has upon the entire refrigeration process.


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4.1 Compressors

Compression is the largest energy input into a refrigeration unit. As such, it isimperative that attention be directed at reducing energy consumption on thecompressor(s). Much of the reduction will be the direct result of actions taken

elsewhere in the unit and even in other operating units. The reader is referred tothose parts of this module for guidance. This sub-section looks at thecompressor(s) specifically.

The three types of compressors found in refrigeration units are screw,reciprocating and centrifugal. They operate on different principles and thesedictate, to varying extents, the type and amount of fuel savings that can beachieved. The comparison between the different compressors is shown in TableE.1 Appendix E.

The fundamental problem with refrigeration compressors is that they are all

designed to work best at one point. The unit design point is the maximumoperating temperature that will be experienced at the plant. The climate inWestern Canada dictates that the compressor(s) could be working at a pointconsiderably away from that design, with the consequent loss of energyefficiency. Every effort should be made to take advantage of the optimumoperating point which is attained by minimizing the refrigeration flow andmaximizing the suction pressure.

The following pros and cons of the main compressor types assume that themachines are operating at design efficiency. Opportunities for fuel savingsthrough maintenance repair are discussed in other sections.

Refrigerant Flow Control

The actions taken to make the refrigeration unit more efficient generally result ina reduction in the circulation of refrigerant. Operators should therefore directattention towards greater refrigerant flow control.

Where there is automatic flow control, such as through engine speedmodulations, there is no real activity required by operators except to ensure thatthe desired control is achieved. Where flow control is manually adjusted, this willgenerally be done using variable pockets and/or unloaders. Since there will be

fluctuations in the plant process and the weather, there will be a constantlychanging process demand. To attempt to attain very tight control would requirean inordinate amount of operation attention. It is preferable to adjust the pockets(and unloaders) at a setting that will ensure all fluctuations can be handled butstill result in reduced energy consumption. While the savings are less, so is therequired attention.


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Some compressors heat their refrigerant in the suction scrubber in order toprevent liquids build-up in the scrubber and possible carryover into thecompressor. While this prevents damage to the compressor, it does result inincreased fuel use since it superheats the compressor feed and adverselyimpacts the isentropic compression of the refrigerant.

It is recommended that the use of heating coils in the compressor suctionscrubber be minimized wherever possible, consistent with safe operation of thecompressor.

4.2 Refrigerant Condenser

The propane condenser determines the discharge pressure on the compressorand thus the power requirements. Nearly all propane condensers in WesternCanada use aerial cooling. A few have water cooling and there can be benefits

to using a combination of both air and water as cooling media (See Appendix D).

It is critical that the condenser be operated as efficiently as possible. Fouling ofthe condenser or impurities in the circulating refrigerant are two areas wherecondenser performance can be seriously impacted. They are discussed in moredetail in Appendix F.

The condenser directly influences the discharge pressure of the compressor. Assuch, the condenser should be monitored constantly. The following energymanagement action items are recommended.

Monitor the temperature approach. Compare the outlet temperatureversus the ambient air temperature when the condenser has just beencleaned. Using this as the basis of comparison, measure the temperatureapproach regularly. A widening temperature gap is an indication that thecooling efficiency is declining. Note that precipitation will improve thetemperature approach.

Ensure that the fan pitch is appropriate. If the fan motor is controlled by aVFD, set the fan blades at the optimum pitch and allow the VFD to adjustthe speed and hence the air flow. Normally, it is desirable to maximize theair flow until the minimum system operating pressure is reached.

Ensure adequate air flow. Check the gap between the fan blade tip andthe fan housing if more than about one inch, backflow of air will occur,which reduces cooling. Check the fan blades for damage. Ensure thatthe fan belts are not worn or damaged.

Clean the condenser bundle. It is recommended that aerial coolers becleaned after significant fouling events (such as poplar fluff) but prior tohot weather so that there is maximum cooling capability over the summer.


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Avoid practices that lead to fin damage. Use of high pressure water spraycan lead to fin damage, which reduces the effective area of thecondenser.

Avoid the use of water spray to augment cooling. If absolutely necessary,consider spraying underneath the bundle and have the water drawn up

through the bundle. Consult the cooler manufacturer prior to using waterspray.

Check to make sure that hot air from another part of the plant process isnot being drawn into the condenser fan. This phenomenon can bedemonstrated using smoke bombs or even sensed by positioning oneselfaround the perimeter of the cooler underneath the bundle. If there is airbeing drawn in, determine the source of the hot air and investigate ways toresolve the problem. In some plants, the outlet plenum of the offendingcooler is extended approximately 6-8 feet.

Check the refrigerant purity on a regular basis and when there is any

suspicion that purity has changed. Indications of a change can be quicklychecked by plotting the temperature and pressure for the condenser,economizer and chiller versus the expected curve (for pure propane thatwould be Figure F.1 or the curve for the actual and for industrial-graderefrigeration). Preferably, analyse the refrigerant. If possible, ask thesupplier for the composition.

Check the pressure control setting on the receiver (also called theaccumulator). In order to keep sufficient pressure in the refrigerant circuitfor maintaining circulation, a small stream of hot refrigerant vapour isbypassed around the condenser to the receiver and flow-controlled by the

pressure in the receiver. Ensure that the pressure setting is not higherthan needed. (This presupposes that the minimum discharge pressurerequired/allowed by the circuit is known.)

4.3 Economizer

An economizer can significantly reduce energy needs in a refrigeration unit. Itremoves vapour from the refrigeration circuit at an intermediate pressure, therebyreducing the amount that would have to be compressed up from the suctionscrubber pressure.

Not all units will have an economizer. As discussed in Section 4.1, somecompressors are not configured to allow the installation of an economizer.Others have not been so equipped due to capital expenditure considerationduring the original plant construction.

The following energy management opportunities are suggested concerning theoperation of the economizer.


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Depending upon the thermodynamic properties of the process gas and the salesgas compression, significant energy savings could be realized if the chiller wereconsidered in conjunction with the low temperature separator (LTS). Theoptimum energy use would be achieved when those two vessels were operatedas one, by taking advantage of the Joule-Thompson effect and the process gas

was chilled at stream pressure and then flashed down to the optimum pressure inthe LTS. The reader is referred to Appendix C for a discussion of thisopportunity.

Before the optimum chiller/LTS conditions could be implemented, the followingenergy management studies would have to be done. These action items aregenerally outside the scope of operators. They usually involve computersimulations, or similar complex calculations. Obviously, if product values changesignificantly, it would be necessary to repeat the above steps using the new data.

Determine the optimum process temperature,

Determine the optimum LTS pressure, and

Determine the chiller temperature that gives the optimum temperature inthe LTS at the optimum LTS pressure.

The reader is referred to Section 4 for more detail regarding these studies.

Assuming that the operators are provided with the target temperature andpressure:

Operate the chiller level control valve in order to achieve the desired target

conditions in the LTS.

4.6 Refrigerant Circuit Piping

The piping in the refrigeration circuit is sized to handle the original design flowrates. Subsequent to plant start-up, conditions could have changed considerablyand the existing piping may be causing a loss of energy efficiency.

Restricted piping causes unwanted pressure drops. These, in turn, cause

generation of vapour according to the refrigerant vapour-liquid equilibrium curve.Vapour reduces the amount of liquid refrigerant to the chiller, which reduces theamount of possible chilling. Ultimately, it could limit the circulation of refrigerantat low circuit pressure by causing the chiller LCV to go wide open, thereby losingcontrol capability.It is necessary to determine the minimum compressor discharge pressure thatthe refrigeration unit can handle, since the unit works more efficiently at lowerdischarge pressures (for the same suction pressure). There are a number of


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limiting conditions, the first-encountered of which dictates the minimum operatingpressure.

The chiller control valve goes wide open, thereby losing the ability tocontrol.

The pressure drop through the piping (and control valve) circuit is greaterthan the desired discharge pressure. The only way to balance thedelivered pressure and required pressure drop would be to reduce theflow rate of refrigerant.

The reduced pressure causes refrigerant vapour breakout upstream of thechiller thereby reducing the amount of refrigeration capable in the chiller.

The lower pressure causes excessive gas velocity through the oil-refrigerant separator on the compressor discharge such that the oil is notentirely separated.

The first three limits are readily observable in normal operation. The last,however, shows no immediate signs of a problem. However, there will be a lossof oil inventory which is the first indication of trouble. In the long-term, the lost oilwill coat the internal side of the tubes of the condenser and potentially even theexternal side of the tubes of the chiller. The oil coating will reduce heat transferand seriously impact performance compressor discharge pressures will riseand the refrigeration effect will drop.

The following action is recommended in order to determine the operatingboundary:

Determine the minimum discharge pressure on the refrigerantcompressor. This is best done by consulting with the designer of the unit(to first determine conditions which absolutely must be avoided) andexperimenting to see how far the pressure can go without adverselyaffecting performance. While Facility Engineering should take the leadrole in this exercise, operator input is extremely important and useful.

Once the minimum operating pressure has been determined, the followingactions, by the operating, are recommended:

Ensure there are no unwanted pressure restrictions in the refrigerantcircuit. Some restrictions such as level control valve pressure drops intothe economizer and the chiller are desired. On the other hand, controlvalves should not be wide open. Inadvertent pressure drops caused bypartially-closed valves or material in the piping should be eliminatedwherever possible.


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5. Facility Engineering Input

The foregoing review of energy management in refrigeration units has illustratedthat there is a wide range of opportunities for savings and/or improved productvalue. The ability for a plant to take advantage of them falls into three

categories:

immediate, low/no capital investment,

minor capital investment,

major capital investment.

Experience has shown that roughly half of the savings identified in energy auditsare in the first category. A well-run unit will accrue significant savings. Thisrequires mostly operator attention and this module has been prepared with that

goal in mind.

On the other hand, capital investment coupled with good operating practices will greatly improve the energy efficiency of the unit. While facility engineeringshould take the lead role, operators can provide valuable input in view of theirexperiences and observations from running the specific, or a similar, unit.

This section discusses the role that facility engineering can play in ensuring thatthe refrigeration unit is operated as efficiently as possible.

5.1 Strategic Goals and Operating Parameters

The conventional refrigeration unit consists of a simple circuit no economizerand no sub-cooler. While that configuration involves the least capital investment,it forgoes substantial savings in operating costs and/or liquid recovery. Retrofitswill bring about those savings, but at a much higher installation cost than if thesame configuration had been incorporated into the original design.

When considering a new refrigeration unit, or modification of an existing unit, it isimportant to establish the strategic goals and the consequent operatingparameters required to meet those goals.

Clearly establish the goal(s) of the unit. Is the unit to be operated forhydrocarbon dew point control or liquid recovery?

Clearly determine the operating parameters. Dependent upon the unitgoals, what range of operating conditions are desired? Generally, with awide the range of conditions, there will be deterioration of efficiency at


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some point within that range. It is also important to know how thoseconditions will change over time.

Design with maximum unit complexity consistent with unit goals. Ifincreases in complexity such as installation of an economizer areforecasted, try to avoid costly retrofits by increased capital investment at

the time of original design.

Ensure that staff are well trained in energy management concepts withrespect to the process and to the equipment.

5.2 Retrofits and New Units

The following actions should be considered, primarily by facility engineers, whendesigning a new refrigeration unit or modifying an existing one. They have beenlisted by equipment. Where appropriate, facility engineering should seek

operator input into the specific design.

Compressor

Determine the minimum discharge pressure on the refrigerantcompressor. This is best done by consulting with the designer of the unit(to first determine conditions which absolutely must be avoided) andexperimenting to see how far the pressure can go without adverselyaffecting performance.

Economizer

Consider the feasibility of installing one, especially if the compressor(s)can accept an intermediate feed. Space, especially in small units, is alsoan important factor, due to the normally-congested nature of therefrigeration skid.

Sub-Cooler

If the plant is considering the installation of a sub-cooler, and there aremultiple choices for the cooling medium, consider the options from thepoint of view both the refrigerant circuit and the rest of the plant. In otherwords, where is the best place within the entire facility for taking thecooling stream(s)?

Chiller/LTSDetermine the optimum process temperature. Usually, that is dictated bywhether the plant is cooling the process gas in order to achievehydrocarbon dew point control or whether the goal is NGL recovery. Inthe first case, the temperature is generally around -17C to -10C (0F-14F). In the second case, the will often be run at -30C or colder.


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Determine the optimum LTS pressure. That is typically done using aphase envelope and an overall holistic study wherein compressionefficiencies (on the refrigeration unit and on the process side), relativeproduct values for sales gas and NGL/NGL individual components areconsidered. If there is an incentive to choke the process gas into the LTS,

it will be necessary to determine the chiller temperature that gives theoptimum temperature in the LTS at the optimum LTS pressure.

Piping

Ensure there are no unwanted pressure restrictions in the refrigerantcircuit. Some restrictions such as level control valve pressure drops intothe economizer and the chiller are desired. Inadvertent pressure dropscaused by partially-closed valves or material in the piping should beeliminated wherever possible.

When modifying the unit, ensure adequately-sized piping and the pipingruns. Obviously, there will be pressure drops in any piping as long as

there is a flow. But where there is some flexibility in the design andproject economics, consider larger-diameter piping. The pipingconfiguration such as rises in elevation - is important also. Once vapourhas broken out of the mix, such as due to a rise in piping elevation, itdoes not turn back into liquid on a subsequent drop in elevation unlessthere is a change in the energy of the fluid (i.e., from cooling orcompression).


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Appendix APropane P-H Diagram

This appendix illustrates how the thermodynamics of a refrigeration circuit, in thiscase propane, can be used to analyze the operation of the propane side of the

unit. In doing so, the concepts discussed in the text of the module are illustratedgraphically. For more information regarding refrigeration units, the reader isreferred to the GPSA Engineering Data Book, 11th Edition, Chapter 14.

While this investigation can be done using a simulation package and will producemore precise results, the use of the P-H diagram is favoured in this case for tworeasons:

The graphical method illustrates the concepts more clearly,

The method has much more availability for operating staff.

A.1 Simple Refrigeration Cycle

The simplest refrigeration cycle consists of a compressor, condenser, expansionvalve (commonly called a JT valve) and the chiller. Figure A.1 outlines this cycle.


Compressor

Receiver

Expansion Valve

Chiller

Condenser

Process Gas

Figure A.1Simple Refrigeration Circuit


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The calculation basis of the refrigeration cycle is the P-H diagram which outlinesthe thermodynamic properties of the refrigerant. (The x-axis is the enthalpy - thesymbol for which is H and the y-axis is pressure.) Figure A.2 shows thisinformation for propane2.

Figure A.2Pressure-Enthalpy (P-H) Diagram for Propane


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For this loop, the following conditions apply:

the ambient temperature is 80F,

the circulating rate of propane is 40,000 lbs/hour, and

the chiller operates at 20 psia (-32F).

The (reciprocating) compressor discharge conditions are 200 psia and 163F.The compressor has two stages. Assume that the starting point is the chiller,which is operating at 20 psi absolute, (-32F). The outlet of the chiller is 100%vapour. The enthalpy of the propane is 413 BTU/lb.

There is a 1.5 psi drop between the chiller and the compressor suction. Since nowork is done, the total enthalpy of the propane remains at 413 BTU/lb but there isa drop in temperature of roughly 3 degrees Fahrenheit, i.e., to -35F.

Normally, compression in screw and reciprocating machines is isentropic.Therefore, the theoretical discharge temperature would be 125F. However,since the compressor does not operate at constant entropy (the sloping lineslabelled s show lines of constant entropy), the discharge temperature is higher.In this case, the discharge temperature is 163F.

This information can be used to determine the compressor horsepower and theoverall efficiency of the compressor. The enthalpy of the propane into thecompressor is 413 BTU/lb. The theoretical enthalpy of the discharge is 462BTU/lb (200 psia and 125F) and the actual enthalpy is 480 BTU/lb (200 psia and163F).

The compression ratio of the compressor is 200 / 20 = 10. This means that thecompression ratio for each of the two stages is 3.16. This is a relatively highvalue and potential for rod loading/rod reversal issues should be investigated.

The brake power to the compressor is 480-413 = 67 BTU/lb. For the circulatingload, the total power is 40,000*67 = 2,680,000 BTU/hour, which is equivalent to1,053 horsepower. The theoretical power input is 462-413 = 49 BTU/hour. Theoverall efficiency of compression is therefore 49 / 67 = 73.1%.

Note, strictly speaking, the enthalpy of the circulating oil (if applicable) must be

taken into consideration. The net result of the circulating oil is to reduce thedischarge temperature. This is especially the case with screw compressorswhere the amount of heat absorbed by the oil can be very significant.

Typically, there is roughly a 10 psi pressure drop between the compressordischarge and the condenser, which is therefore operating at 190 psia. The dutyof the condenser is based on the assumption that the outlet of the condenser is


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100% liquid. Therefore, the outlet has an enthalpy of 312 BTU/lb, for an overallduty of 480 312 = 168 BTU/lb, or 6,720,000 BTU/hour.

The temperature approach of the condenser is the difference between thecondenser outlet temperature and the ambient temperature. Since the outlet

pressure is 190 psia, this is equivalent to an equilibrium temperature of 99F.The temperature approach is therefore 99 80 = 19 degrees Fahrenheit, whichis typical of design values, although in many units much better performance isachieved.

The chiller conditions are achieved by dropping the pressure through theexpansion valve, which is the level control valve on the chiller. Again, becausethere is no work done, there is no loss of enthalpy in the propane as it goesthrough the LCV. Graphically, this is shown by the vertical line dropping from thecondenser outlet (P=190 psia, H=312 BTU/lb) to the chiller inlet (P=20 psia,H=312 BTU/lb.

At the chiller conditions, the enthalpy of the propane when liquid is 234 BTU/lband the enthalpy of propane vapour is 413 BTU/lb. The difference 179 BTU/lb

is the latent heat of vaporization of propane at 20 psia. The weighted enthalpyof the vapour-liquid mixture entering the chiller is 312 BTU/lb. The proportionthat is liquid can be calculated by the following formula

(Hvapour Hmixture) / (Hvapour Hliquid)or

(413 312) / (413 234) = 56.42%

Nearly half of the propane entering the chiller does so in the form of vapour.While the low temperature of the vapour will contribute to the chilling of theprocess gas, the amount is negligible compared to the chilling achieved byvaporizing the liquid propane entering the chiller. In other words, the amount ofchilling, or refrigeration effect, is related to the amount of liquid propane enteringthe chiller. The refrigeration effect is therefore

40,000 lb/hr * 0.5642 * (413 234) = 4,040,000 BTU/hour

or40,000 * (413 312) = 4,040,000 BTU/hour

Two terms are frequently used with reference to refrigeration units:

Tons of Refrigeration. This is defined as the amount of heat required tomelt 1 ton of ice in 24 hours. It is, by definition, equal to 12,000 BTU/hour.In our example, the tons of refrigeration total 4,040,000 / 12,000 = 336. 7TR.


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Coefficient of Performance. The coefficient of performance (COP)equals the amount of chilling divided by the amount of work added in thecompressor. In our example, the COP is 4,040,000 / 2,680,000 = 1.507.

It can be seen from Figure A.3 that any steps that reduce the compression ratio

will increase the proportion of liquid entering the chiller. Fuel requirements willdecrease due to the lower compression ratio and the need to circulate lesspropane in order to achieve the same amount of chilling.

A.2 Economizer and Sub-Cooler

The compression power requirements can be reduced by adding either aneconomizer, or a sub-cooler, or both. Figure A.4 shows the revised configuration

in this case, two-stage refrigeration - and Figure A.5 illustrates the impact of thenew economizer on the P-H diagram.


Compressor

Receiver

Expansion Valve

Chiller

Condenser

Process Gas

Economizer

Subcooler

Figure A.4Complex Refrigeration Circuit


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Economizer.

In this example, the compressor consists of two stages and can thus handle aneconomizer. The economizer pressure has been set at 70 psia. The enthalpy ofthe saturated vapour and the saturated liquid are 432 BTU/lb and 270 BTU/lb,

respectively. The enthalpy of the liquid out of the condenser/receiver is 312BTU/lb (see previous sub-section). Therefore, the fraction of liquid in thepropane entering the economizer is (432-312) / (432-270) = 74.07%.

The vapour removed in the economizer is returned to stage 2 of the compressor.Power/fuel savings are achieved because the economizer vapour does not haveto be compressed from chiller pressure. From the viewpoint of the compressor,the optimum discharge pressure on stage 1 would be roughly 63.3 psia. Notethat the economizer pressure is 6.7 psi higher, that difference being taken up bythe back pressure control valve on the gas off the economizer. This observationwill be discussed later.


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Since the chiller and compressor suction conditions are identical to the simplerefrigeration cycle scenario discussed in the previous subsection, the change inenthalpy of the propane vapour into stage 1 is 67 BTU/lb. However, the fractionof propane entering the chiller has changed dramatically. The vapour that wouldhave been generated from dropping the pressure from 190 psia (in the receiver)

to 70 psia (in the economizer) has been removed.

The enthalpy of the propane liquid entering the expansion valve is the same asthat of the liquid in the economizer, 270 BTU/lb. Therefore, the fraction of liquidentering the chiller is (413-270) / (413-234) = 79.89%.

Prior to the use of the economizer, the refrigeration effect was estimated to be4,040,000 BTU/hour. Since the latent heat of evaporation in the chiller is 413 234 = 179 BTU/lb, the required amount of liquid propane in the chiller is 22,570lb/hour. In view of the fraction of liquid entering the chiller, the required amountof propane circulation is:

22,570 / 0.7989 = 28,251 lb/hour via the chillerplus

28,251 / 0.7407 28,251 = 9,890 lb/hour removed in the economizer.

The total propane flow is 38,141 lb/hour, whereas the original propane circulationrate was 40,000 lb/hour. The total power required in the compressor is

28,251 lb/h*67 BTU/lb+9,890 lb/h*24.6 BTU/lb = 2,136,111 BTU/hr = 840 bhp.

The installation of the economizer reduced the overall propane flow by 4.4%, butreduced the power required by 20.3% (from 1,053 bhp to 840 bhp).

As indicated earlier, stage 2 of the compressor would have an inlet pressure ofabout 63.3 psia, but the economizer was being operated at 70 psia. Assume thatthe economizer pressure could be reduced to 65 psia by opening the backpressure control valve. Repeating the calculation, the new power required wouldbe 834 bhp a reduction of 6 bhp, worth $3,120/year in fuel at $6.00/GJ.Interestingly, there is a small increase in the total amount of propane beingcirculated. However, the increase is in the economizer off gas and there is adecrease in the chiller off gas. The savings in stage 1 power (due to reducedflow) makes up for the small increase in stage 2 power (caused by highercompression ratio).

The savings in a real situation would depend upon the economizer pressure levelrelative to the compressor interstage pressure. For instance, an economizerpressure set at design summertime levels would be at considerable variancefrom the interstage pressure when running in the winter.


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As an example, a 2-stage compressor operating at a suction pressure of 20 psiaand a discharge of 250 psia would have an economizer pressure in the range of70-71 psia. In the winter, when the discharge could be as low as 120 psia (thisvalue would depend upon a variety of factors but could be determinedexperimentally by the plant), the economizer would be at roughly 50 psia.

Sub-Cooler. A sub-cooler increases the proportion of liquid entering the chillerby shifting the equilibrium line into the sub-cooled propane region (to the left ofthe saturated liquid line). In the experience of this documents author, a typicalsub-cooler will deliver 10-20 degrees Fahrenheit cooling. For this exercise, 15degrees is assumed. It is further assumed that the sub-cooler is installed inconjunction with, and after, the economizer. Graphically, the sub-cooler is shownin Figure A.6.

The economizer pressure is 65 psia. Therefore, the equilibrium temperature is28F. With the installation of a sub-cooler the propane going to the expansion

valve would be at 13F. This means the enthalpy of the propane into theexpansion valve is 259 BTU/lb and the fraction of liquid entering the chiller is86.03%.

As a result of installing a sub-cooler, the compressor power required is 786 bhp,a reduction of 5.8% from the economizer case.

A.3 Economics

In summary, the installation of an economizer and sub-cooler has the following

results:

The compressor power requirements dropped 267 bhp, from 1,053 bhp to786 bhp, or 25.4%.

The total propane flow dropped by 10% from 40,000 lbs/hr to 36,016lbs/hr.

The propane condenser duty is correspondingly reduced by 10%. Interms of power requirements, this represents a reduction in condenser fanpower of approximately 100 100 * (.9004 3) = 27.0%. Based on a typicalcondenser design, the original installed condenser power requirement was

about 160 bhp. Power requirements on the condenser would be reducedby 0.27*160=43 bhp (32 kW) on the fan motor output.


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At an engine firing efficiency of 33.5% (i.e., 7,600 BTU fired/bhp) and a fuel valueof $6.00/GJ, the savings in engine firing would be:

267 bhp * 7,600 BTU LHV/bhp-hr * 8,760 hr/yr * 1.1 HHV/LHV

* 1,055 J/BTU / 1,000,000,000 J/GJ * 6 $/GJ HHV = $123,770/year

Other economic factors to consider are the following:

The fuel saved would be equivalent to approximately 1,082 tonnes ofCO2 /year. At a value of $15.00/tonne, the value of the emissions is$16,230.

The electrical savings on the condenser are 43 bhp, which is equivalent toabout 47 input hp. At a cost of $0.10/kWh, the cost of electric powerwould be $30,700 annually.

A reduction of 267 bhp in engine use will result in a reduction in enginemaintenance. At a rate of $60/year/bhp, the savings inmaintenance/spare parts would be $16,020 annually.

If considering a retrofit installation of an economizer/sub-cooler, the savings infuel, emissions, electrical costs and engine maintenance total $186,720.

If considering a new refrigeration unit, the above-mentioned costs would apply.In addition, there would be a reduced cost of roughly $1,000/hp for theequipment and $1,000/hp for the installation cost. The total savings in capital

expenditure are thus $534,000 plus the $186,720 in annual operating expenses.


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Appendix CChillerLTS Coordination

Considerable energy savings can be achieved by setting operating conditions inthe chiller and the low temperature separator (LTS) at the optimal values. This

appendix outlines a conceptual approach experience has shown that eachplant and/or gas is unique and no definitive and all-inclusive quantitativerecommendations can be made.

The conventional approach is to cool the process gas in the chiller and flash it inthe LTS, which would be at a pressure not much different from the process sideof the chiller. The proposed approach is to partially chill the gas and then chokeit prior to flashing it in the LTS. In other words, there will be a significantdifference in pressure between the chiller and LTS.

Due to the complexity of vapour-liquid equilibrium calculations, this exercise is

best done by a computer simulation package.

The starting point for this analysis is the phase map of the process gas. FigureC.1 shows the phase map for three hypothetical gas feeds, the major variationbeing the amount of methane in the stream, the rest of the components beingadjusted accordingly.

Refrigeration Unit Feed - Phase Map

0

200

400

600

800

1000

1200

1400

1600

1800

-300 -250 -200 -150 -100 -50 0 50 100 150

Temperature, F

Pressure,psia

All Liquid

All Vapour

Dew Point

Bubble Point

85% C1

75% C180% C1

Figure C.1

Refrigeration Unit Feed Phase Map


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On the left hand side of the graph is the bubble point curve. At temperatures tothe left of the curve the fluid is all liquid. On the right of the graph is the dewpoint curve, and at temperatures to the right of the curve, the fluid is all vapour.

Note that the bubble point curves (where the fluid starts to vaporize) are

essentially identical but the dew point curves (where condensing starts) are quitedistinct. As expected, the richer the gas the higher the temperature at whichliquid starts to condense.

At temperatures that lie between the bubble and dew point curves, there is amixture of vapour and liquid. As with the case of the P-H diagram for propane(see Appendix A), when in a two-phase region, the closer the point is to the dewpoint curve the smaller the fraction of liquid in the mix. It should be pointed outthat the fraction in this case is the molar fraction, whereas in the P-H diagram,which is shown in Appendix A, the fraction can be either molar or mass (since itis for pure propane).

Figure C.2 illustrates the effect of choking the process gas as it exits the chiller.

NGL Recovery by LTS Choke Valve

0

200

400

600

800

1000

1200

-150 -100 -50 0 50 100

Temperature, F

Pressure,

psia

Bubble Point

Dew Point

20 mol% Liquid

30 mol% Liquid

40 mol% Liquid

Flashing Through Choke Valve

Figure C.2

NGL Recovery by LTS Choke Valve


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smaller condenser duty, as well as changes in the reboiler duty on the de-ethanizer and any subsequent fractionation towers.

On the other hand, it will be necessary to recompress the gas coming off the LTSback up to the line pressure that is upstream of the LTS choke valve.

In determining the optimum LTS pressure, it is necessary to factor in the cost ofrecompressing the sales gas back to the stream pressure, the savings inrefrigeration compression and the net change in value of sales gas versus NGL(or propane, butane, etc.)

It is recommended that any study of choked LTS operation include an estimate ofthe hydrocarbon dew point of the sales gas expected at the new LTS conditions,in order to ensure that the sales gas specification is still met. If necessary,repeat the calculations.


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water condenser uses an exchanger whereas an aerial cooler requires anenergy-consuming fan (and associated maintenance costs).

It has been estimated that water cooling reduces compression requirements by20% compared with aerial cooling, all other conditions being equal.3 There is

even the possibility of twinning water cooling (for summertime) with aerial cooling(for wintertime).

The water could be a cooling water stream but that requires a cooling tower, i.e.,energy input. As an alternative, water ultimately going to boiler feedwater make-up, could be used. The heat taken out in the condenser would now be retainedin the overall plant energy envelope rather than rejected into the atmosphere.

D.2. Refrigeration Lean Oil Absorption

Lean oil absorption is sometimes used for recovery of NGL components. A

detailed description of the process is outside the scope of this present document.Suffice it to say that the effectiveness of the lean oil absorber can be improved byreducing the temperature of the absorber. Low temperatures reduce thevapour/liquid equilibrium values, making the removal of heavier components fromthe gas easier. This means that lower lean oil rates are needed.

When a refrigeration unit is operated in conjunction with a lean oil absorber, therefrigeration unit chiller can be used to cool the lean oil and at the same time toprepare the NGL for absorption by partially dropping out NGL this also reducesthe amount of lean oil circulation. On the other hand, lower lean oil temperaturesmean that there has to be increased firing on the lean oil regeneration tower.

Optimization of an integrated refrigeration/lean oil absorption unit entails a trade-off between increased power requirements for propane compression due to lowerchiller pressures and decreased fuel firing on the lean oil regenerator.

D.3 Cascaded Refrigeration

Deep-cut fractionation of the sales gas, in order to remove ethane from the gas,is often done using a turbo-expander. An alternative is to use cascadedrefrigeration, which involves two refrigerants. The first, such as ethane, is usedto achieve the extremely low temperatures required for ethane recovery from the

gas. The second, such as propane, is required in order to achieve coolingtemperatures low enough to condense the first refrigerant. Both refrigerationloops operate as conventional units, except that the ethane loop condenser is thepropane loop chiller. For more details on this process, the reader is referred tothe GPSA Engineering Data Book, Chapter 14.


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the third choice, the stage 1 suction pressure drops to 148.8 kPaa. Powerrequirements are 48.1 bkW, or 88.2% of the original.5

E.3 Centrifugal Compressor

Like virtually all centrifugal machines, as the flow increases the powerrequirements also increase6. At the same time, the delivered head declines.Therefore, in order to meet the extreme summertime demands, where maximumhead and maximum refrigerant circulation rate are needed, the machine isconsequently oversized for the rest of the year.

Figure E.1 shows the performance curves for the first stage of a centrifugalcompressor handling propane. The major points of flow control are illustrated inthe graph.

Flow control on centrifugal machines is best done by slowing the machine. In theexample below, the speeds are 1 and 2.

Assume that the flow rate of the machine is 2,400 ACFM. The compressor willline out where the pressure developed by the machine equals the system curve,which is the process pressure that the compressor must overcome. In this case,the system curve is the vapour pressure of the propane at the condenser outlettemperature. At 2,400 ACFM the head developed is roughly 18,420 feet and theefficiency of compression is 77.4%. The required brake horsepower is 486.0 hp.

Reducing the speed (from Speed 1 to Speed 2) causes the compressor flow tore-equilibrate at 2,070 ACFM and a head of 18,420 feet. The efficiency curve

shifts as a consequence of the speed change. There is a loss of efficiency butless than the loss if the flow reduction had been achieved by choking in thedischarge valve at the original compressor speed. An efficiency of 76.3% isassumed.

This results in a power requirement of 426.2 bhp, which is 87.7% of the basecase whereas the new propane circulation is 86.2% of the old. Note, whenmaking the speed reduction, be very careful of avoiding compressor surge.

Figure E.2 investigates the power savings if the ACFM flow is kept constant butthe suction pressure is reduced in order to reduce the mass of propane.

If the suction pressure is reduced, the required polytropic head will want to rise toreach the discharge pressure. Being a centrifugal machine, the compressor willcut back flow and could pass the surge line a situation which must be avoidedat all costs.

In order to prevent surge, the operators should keep the flow 10-15% higher thanthe surge point and drop the pressure in very small steps. In this case, a change


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It should be noted that this compressor had a second section and any changes insuction pressure would have to take into consideration the entire machine.However, this example was only to illustrate the concept that would be supplied.

In summary, reduction in suction pressure is not as effective in saving energy as

reducing the compressor speed. Of course, if there is no ability to change thespeed, suction pressure reduction should be considered. In any event, operatorsmust be fully cognizant of compressor surge and take action in carefully-monitored steps.


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Table E.1Compressor Comparison Chart

Compressor Type Advantages Disadvantages- Most often used in small

refrigeration units

- Must circulate high volumes

of oil which must be preventedfrom entering the condenser- Can run at very low suctionpressure and can deliver veryhigh compression ratios

- Designed for a specificcompression ratio there is aloss of efficiency whenoperated above or belowdesign

Screw

- Capable of having aneconomizer installed

- Most often used in mediumsized refrigeration units

- Must have multiple stages(two or three) to handlesummer operation

- Versatile, commonly usedand well understood - Not always capable of havingan economizer installed

Reciprocating

- Can be efficient over a widerange of operating conditions

- Tight flow control is notalways possible whenrefrigerant demand is reduced

- Most often used in mediumand large sized refrigerationunits

- Operating principle runscounter to a refrigerationcircuit. If refrigerant dischargedrops, due to lower condensingtemperature, the compressortries to supply more flow,whereas less refrigerant is

required.- Can circulate large volumesof refrigerant

- Flow control can be achievedby reducing the speed of thecompressor but the driver willlose efficiency. This is mostsignificant with turbines.

- Can handle flows fromeconomizers

- Efficiency can decreasesubstantially as the circulationrate varies from design

Centrifugal

- Critical to circulate at a rateabove the surge point andbelow the maximum or choke

flow


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Appendix FCondenser Performance

F.1 Fan and Bundle

The condenser is designed to condense propane during the hottest days of theyear. For much of Western Canada, the design air temperature is 32.2C (90F)although some sites in the northern section use 26.7C (80F). When assessingthe performance of an aerial cooler the most important parameter is thetemperature approach the difference between the condenser outlettemperature and the ambient air temperature. Typically, the design temperatureapproach is 10-15 degrees Celsius (18-27 degrees Fahrenheit). In other words,if the ambient temperature is 32.2C, the condenser outlet temperature will be42.2-47.2C.

For a propane condenser, those outlet temperatures would translate to pressures

of 208.3-233.3 psia. Even a 2 degree Celsius decline in approach temperature(i.e., to 12-17 degrees Celsius) due to fouling, etc., would add 10-12 psia to thedischarge pressure.

Condensers can lose effectiveness for a wide variety of reasons:

Fouling. Poplar fluff, chaff, dust, animal matter can lead to considerablefouling. Water on the tubes will also leave scale and cause corrosionwhere there are different metals for the fins and tubes.

Damage. Care should be taken so that the fins on the tube bundles arenot damaged.

Fin Delamination. This can occur when cold water is sprayed onto a hotbundle. The use of water spray is a common practice in an effort toimprove cooling. The result is short-term and can lead to later problems.The water causes the fin to pop off the tube, leaving a small gap whichreduces heat transfer from the tube into the air stream via the fins.7Unfortunately, once it occurs, delamination cannot be reversed.

The damage occurs when cold water is sprayed on the hot tubes. Someplants attempt to get around this by spraying the water underneath the fanand being drawn up through the bundle. In this case, the cold watercontacts colder tubes minimizing delamination. It is recommended that

the aerial cooler manufacturer be contacted when considering the use ofwater spray.

Reduced Air Flow. This can occur if the fan pitch has not been properlyset. Very often, the pitch is adjusted before winter in order to minimize airflow during very cold weather and readjusted for summertime air flow.

Inefficient Air Flow. This occurs when there is a large gap between thefan blades and the fan housing. The result is an actual downflow of air


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through the fan. Damage to the fan blades can also cause this flowreversal.

Proximity to Other Coolers. Studies by cooler manufacturers and someplants have shown that air exiting an adjacent cooler (or a heat source)can be drawn into the inlet of an aerial cooler.

F.2 Refrigerant Purity

It is important to know the purity of the circulating refrigerant. The purity affectsthe temperature-pressure relationship in the refrigerant receiver or accumulator;in the economizer (if there is one); and in the chiller or evaporator. Figure F.1illustrates this concept, using propane as the refrigerant.

The operation of the condenser is dictated by the bubble point curve. Theoperation of the chiller is dictated by the dew point curve. For pure propane,

these two curves are identical.

In Figure F.1 the temperature-pressure curve for pure propane is shown in boldfont. The dashed line to the right of the pure propane line is the bubble pointcurve for typical industrial-grade propane as would be used in a plant. (The dewpoint curve lies, coincidently, virtually on top of the pure propane line.) Thedashed line to the left of the pure propane line is the dew point for propanecontaminated by butanes. (In this case, the bubble point curve is virtuallyidentical to the pure propane curve.)


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Propane Temperature-Pressure Equilibrium

-40

-20

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140 160 180 200 220 240 260

Pressure, psig

Temperature,

F

Assumes Patm = 13.5 psia

Pure Propane

Bubble Point - Industrial Propane

Dew Point - Propane with C4's

Figure F.1

Propane Temperature-Pressure Equilibrium

In summary, if the contaminants are lighter than propane (methane and/orethane) the curve shifts to the right, meaning that for a given outlet temperaturefrom the condenser, the compressor discharge pressure will be higher. Somereciprocating compressors cannot handle significant quantities of impurities

during warm weather because they are limited in the compression ratio that theycan deliver.8

On the other hand, if the contaminants are heavier (butanes and/or pentanes) thepressure in the chiller will be much lower for the same chiller temperature. Thisin turn, significantly affects the compression ratio and thus the power required.

It should be pointed out that, if the contaminants include both butane (andheavier) and ethane (and lighter), the butanes will probably be drained from thecircuit, most likely from the suction scrubber. This could result in an increase inethane concentration and a rise in power requirements.

Contaminants will also affect the amount of refrigerant entering the chiller asliquid, which, in turn, affects the amount of refrigeration that can be achieved.However, this impact is usually very small and can be ignored in most day-to-dayoperation unless the amount of contamination is very large or if the contaminantis very light (such as methane).


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Appendix G

References

The following documents were referenced while preparing this Best ManagementPractice.

http://www.patchingassociates.com/lan/newsletter_10.htm, Patching AssociatesNewsletter #10, March 1999

Improving Operational Efficiency [name withheld] Gas Plant Pilot Audit; prepared for[name withheld]; prepared by CETAC-West; Calgary, AB; February 24, 2003

GPSA Engineering Data Book, 11th Edition, Chapters 14 and 24; Gas ProcessorsSuppliers Association, Tulsa, OK; 1998

Schaums Theory and Problems Thermodynamics for Engineers; by M.C.

Potter and C.W. Somerton; McGraw-Hill Inc., 1995, page 71

Mycom 3.22ep; Screw Performance 3-22; Recip Performance 3.2; MayekawaMfg. Co. Ltd; 1998-11-04.

Endnotes

1

Power consumption and refrigeration effect values found using the Mycom 3.22ep software.2 P-H diagram from GPSA Engineering Data Book, 11th Edition, Figure 24-27, with refrigerationloop overlaid on it in Figures A.3, A.5, A.6..3http://www.patchingassociates.com/lan/newsletters/newsletter_10.htm, March 1999

4 Mycom 3.22ep; Screw Performance 3-22; Recip Performance 3-2; Mayekawa Mfg. Co. Ltd.;1998-11-045 Based upon GPSA calculation techniques outlined in the Engineering Data Book, 8 th, 9th and10

thEditions.

6The word virtually is used because very large centrifugal pumps sometimes have sections of

their pump curves, towards the end of their capacity, where the required power levels off and mayeven decrease.7

http://www.patchingassociates.com/lan/newsletters/newsletter_010.htm8

Mycom 3.22 ep9 GPSA Engineering Data Book, 11th Edition, Figure 14-26.

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