Bomba de Refrigerante036-043_jekel

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3 6 A S HR A E J o u r n a l a s h r a e . o r g A u g u s t 2 0 1 1

Early practitioners of industrial refrigeration systems found they

could increase the capacity of evaporators by supplying excess

liquid refrigerant to the unit (overfeeding). In evaporators congured

to operate with overfeed, the quantity of liquid refrigerant supplied is

greater than the minimum amount required to meet the cooling loads

as it undergoes the phase change from liquid to vapor. In this case, a

mixture of low temperature liquid and vapor leaves the evaporator

and returns to a vessel designed to separate the liquid from the vapor

prior to the vapor being recompressed.

Within limits, the cooling capacity o

an overed evaporator increases due to

the tendency or more o the evapora-

tors interior suraces being wetted with

saturated liquid rerigerant.1,2 Figure 1

shows a simple liquid overed system

typical o those designed and used today

or large built-up industrial rerigeration

systems.

In a mechanically pumped overeed

system, a centriugal pump draws low

temperature saturated liquid rerig-

erant rom a vessel reerred to as a

pumped recirculator, pumped ac-

cumulator, recirculator, or low-

pressure receiver and raises the pres-

sure o the liquid or delivery to one

or more evaporators having a common

rerigerant temperature requirement.

Once pressurized by the pump, the

saturated liquid becomes subcooled

as it leaves the pump discharge and

enters the recirculated liquid supply

line. As individual evaporators call

or cooling, local controls simply open

a liquid eed solenoid valve, which al-lows low temperature pressurized liq-

uid rom the liquid supply line to ow

into the evaporator. Manually adjust-

able hand-expansion (i.e., metering)

valves are used at each evaporator as

a means o balancing the supply o

About the Authors

Todd B. Jekel, Ph.D., P.E., is assistant director and

Douglas T. Reindl, Ph.D., P.E., is director of the

Industrial Refrigeration Consortium and professor

at the University of Wisconsin, Madison, Wis.

By Todd B. Jekel, Ph.D., P.E., Member ASHRAE, and Douglas T. Reindl, Ph.D., P.E., Fellow ASHRAE

Liquid Refrigerant PumpingIn Industrial Refrigeration Systems

TECHNICAL FEATUREThis article was published in ASHRAE Journal, August 2011. Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more informationabout ASHRAE Journal, visit www.ashrae.org.


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A u g us t 2 0 1 1 A SHR A E J our n a l 3 7

liquid ow to individual evapo-

rators throughout the system.

The hand-expansion valves are

adjusted to achieve appropriate

liquid overeed rates as-required

or given evaporator designs.Leaving each unit will be a mix-

ture o saturated vapor produced

by absorbing heat rom the rerig-

eration loads and saturated liquid

that was overed. The two-phase

mixture is carried back to the same

pumped recirculator vessel through

the recirculated liquid return or

wet suction return. The recircula-

tor vessel separates overed liquid

rom vapor and liquid alls to the

bottom o the vessel to be pumped

back out to the evaporators while

the saturated vapor is directed to

the compressors through the dry

suction line. Liquid is made up to

the recirculator vessel rom a high

mass ow rate o vapor-phase rerigerant leaving the evapora-

tor in lb/min [kg/s].

The total liquid rerigerant mass ow rate the pump needs

to deliver is then:

m m OR refrigerant pumped refrigerant min, ,= +( )1 (3)

where mrefrigerant,pumped represents the mass ow rate o liquid

rerigerant the pump must supply to the connected overeed

evaporators expressed in lb/min (kg/s). The term (OR+1) is

commonly reerred to as the circulating rate, Nr. The circu-

lating rate represents the mass ratio o liquid pumped to the

evaporators to the amount o vaporized liquid in the evapora-

tors.1Table 1 shows the required recirculating liquid ow rate

expressed in gallons per minute o liquid ow or each ton o

rerigeration load over a range o saturation temperatures and

circulating rates (OR+1).

What overfeed rate is required for an individual evapo-

rator? Evaporator manuacturers typically publish recom-mended overeed rates based on the units specifc design.

Deviating signifcantly rom the manuacturers recommend-

ed overeed rate can lead to lower operating capacity o the

evaporator. Too low o a rerigerant ow rate will starve the

unit but excessive liquid supply will cause the evaporator

to brine. Excessive supply o liquid rerigerant ow also in-

creases the likelihood o pump cavitation. Finally, excessive

overeeding o liquid to evaporators increases the difculty

associated with returning the unused liquid to the recircula-

tor when the return path involves a vertical riser. In this case,

there is an increased tendency or liquid to accumulate or log

up in evaporators.

Figure 1: Mechanically pumped liquid overfeed system arrangement.

High Pressure Gas

EvaporativeCondenser

EvaporativeCondenser

HighPressureReceiver

Dry Suction

Compressor(s)Centrifugal Liquid

Refrigerant Pump

PumpedRecirculator

High PressureLiquid

KingValve

E

qualizerLine

OverfedEvaporator(s)

Recir

culat

edLiquid

Retur

n(We

tSucti

on)

Recir

culat

edLiquid

Supply

(Pum

pedL

iquid

Line)

T

pressure part o the system to replace the liquid that is evaporated

to meet the rerigeration loads; the recirculator vessel separates

the ash gas ormed as a result o this throttling process just as it

separates the two-phase mixture returning rom the evaporators.

How Much Liquid Refrigerant Needs to Circulate?

For industrial rerigeration systems using ammonia as the

rerigerant, the ow rate o liquid rerigerant delivered by thepump to connected evaporators is relatively low due to the re-

rigerants high heat o vaporization. The total pump ow rate

will depend on the minimum mass ow rate required to meet

the aggregate capacity o connected evaporators and the recom-

mended overeed rate or the evaporators. The minimum mass

ow rate o liquid required to meet the aggregate rerigeration

load is given by:

mQ

hrefrigerant, min

load, total

fg

= (1)

where mrefrigerant,min is the minimum liquid rerigerant that

must be supplied to all connected evaporators to meet theiraggregate load in lb/min (kg/s), Qload,total is the aggregate re-

rigeration load in Btu/min (kW), and hfg is the enthalpy o

vaporization or the rerigerant at the operating pressure o

that suction level in Btu/lb (kJ/kg). The overeed rate (OR) is

the ratio o liquid mass ow rate to vapor mass ow rate leav-

ing the evaporator:1

ORm

m

liquid

vaporwet return

=

(2)

where mliquid is the mass ow rate o liquid-phase rerig-

erant leaving the evaporator in lb/min [kg/s] andmvapor is the


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Recirculated Liquid Flow Rate (gpm/ton)

Circulating

Rate

Liquid Saturation Temperature (F)

60 40 20 0 20 40

2:1 0.1117 0.1164 0.1216 0.1274 0.1340 0.1415

3:1 0.1676 0.1746 0.1824 0.1911 0.2009 0.21224:1 0.2235 0.2328 0.2431 0.2548 0.2679 0.2829

5:1 0.2793 0.2910 0.3039 0.3185 0.3349 0.3537

Multiply by 0.0646 to convert table values to m3/h per kWT.

Table 1: Recirculated gallons per minute per ton of refrigeration.

Figure 2: Single-stage centrifugal pump.3

Discharge

Suction

Volute

Impeller

Impeller Eye

Anatomy of a Centrifugal Pump

At a undamental level, the centriu-

gal pump used or circulating reriger-

ants is similar to the centriugal pump

used to move water or other secondary

uids. Figure 2 shows a single-stagecentriugal pump common or moving

liquids. The main eature o a centriu-

gal pump is the impeller, which rotates

within the pump casing creating a low

pressure zone near its center (the eye).

This area o low pressure draws liquid

into the pump where the rotating impeller increases the kinetic

energy o the uid by accelerating the liquid outward radially

to the impeller tips. As the liquid leaves the impeller tips, its

kinetic energy is at a maximum. The pump housing or volute

surrounding the impeller then takes over to orderly collect the

liquid leaving the impeller. The process o gathering liquid

in the volute converts the kinetic energy o the uid to pres-

sure (potential) energy. The higher pressure uid then leaves

the pump through the discharge line.

Because a rerigerant pump is moving a volatile uid, it is

highly susceptible to cavitation during operation (see What is

Cavitation? sidebar). To reduce the likelihood o cavitation,

liquid rerigerant pumps include design details that dier

rom ordinary water or secondary uid pumps to decrease the

pressure loss through the pump suction.

Liquid Refrigerant Pumps

Beore discussing the operating details o centriugal re-

rigerant pumps, it is important to consider a ew conceptsundamental to their successul operation. One o the most

important concepts is net positive suction head (NPSH). Quite

simply, suction head represents the pressure at the pumps

suction. The term net positive is intended to account or the

balance o positive pressures (static, i.e., height, and absolute

pressure above the vapor pressure o the uid) and negative

pressures (losses) attributable to uid ow. Eectively, NPSH

is the dierence in pressure o rerigerant at the pump suc-

tion and the rerigerants saturation pressure. An NPSH o 0

or a pump attempting to move a volatile liquid rerigerant

indicates that the liquid will ash to a vapor state as it moves

into the pump.Two types o NPSH that need to be considered to ensure

proper pump operation: net positive suction head required

or NPSHr and net positive suction head available or NPSHa.

NPSHr is the minimum net positive suction head required to

prevent the liquid rerigerant rom ashing to a vapor. It is a

characteristic o each given pump and varies with the pumps

operating point (head and ow) as provided by the pump man-

uacturer.

NPSHa is the available net head at the pump suction ac-

counting or those actors that eectively increase head (static

head to the liquid elevation ahead o the pump suction, sub-

cooling o liquid rerigerant within the vessel) and decrease

head (rictional losses, heat gains, and orm losses). To pre-

vent pump cavitation, the NPSH available to the pump must

be greaterthan the minimum requiredby the pump.

NPSH NPSHa r> (4)

I a rerigerant pump is cavitating, several options can rem-

edy the situation. First, determine the pump impeller diameter

rom original equipment installation documentation or rom

the data tag on the pump. Next, read the pressure on the dis-charge side o the pump during operation and compare with

the vessels pressure to determine that the pump is developing

a pressure rise and note the magnitude (estimated by dier-

ence between the gauge reading and the vessel pressure). Then

obtain the pump curve or that specifc model pump and im-

peller diameter (see the next section).

With this inormation, look at the manuacturers pump

curve and determine the pumps ow rate and the required

net positive suction head (NPSHr) corresponding to that op-

erating point on the pump curve. I the pump is cavitating

due to operating out on the curve (toward the right side o

the pump curve), one approach to cure this cavitation is to


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Photo 1: Liquid recirculator package showing excessive ice ac-

cumulation on insulation indicating the failure of the insulation sys-

tem with corresponding increases in liquid refrigerant heat gain.

reduce down (close) the hand-expansion valves on the liq-

uid overed evaporators being served by the pump. Adjusting

down on, or more, hand-expansion valves will increase the

discharge head on the pump; thereby, decreasing its ow rateand the corresponding NPSHr. This process should ocus

on the largest evaporators frst since they have the greatest

impact on liquid demand. I cavitation-ree pump operation

cannot be achieved by this approach and the recirculator is

equipped with a capacitance probe or liquid level sensing/

control, it may be possible to raise the vessels liquid operat-

ing level to increase the available net positive suction head

(NPSHa). However, raising the operating level will reduce

the capacity o the recirculator vessel or surge (i.e., unex-

pected return o liquid) rom the system. Another possible

reason or pump cavitation is excess parasitic heat gain be-

tween the vessel and the pump suction. Visible rost on thesuction piping, as shown in Photo 1 is an indication o a

loss o insulation integrity, which increases the heat gain to

the rerigerant. To remedy this it will require reinsulating the

pump suction piping.

The Pump Curve

A pump curve is a compact, graphical representation o

a pumps perormance (Figure 3). Lets review the basics

o reading a pump curve or a liquid rerigerant pump. In

this case, we will consider an open-drive pump operating

with ammonia (specifc gravity o 0.7). The horizontal axis

o the pump curve shows the pumps developed ow rate

Cavitation is the formation of vapor bubbles within a

pump followed by their rapid collapse. The formation and

collapse of vapor bubbles produces a distinct audible sig-

nature that sounds as if the pump is circulating gravel.Although cavitation can occur in the process of pumping

any liquid, refrigerant pumps are more susceptible to cavi-

tation because uid being pumped is at or near saturation

conditions (i.e., its boiling point). When cavitation occurs,

the pump loses its ability to consistently move liquid and

to develop pressure lift. The loss of pump capacity (ow)

can starve evaporators leading to a loss of refrigeration

capacity. The action of vapor bubbles collapsing in the

pump causes erosion of the pump impeller with subsequent

degradation of pump capacity and efciency. For semi-

hermetic pumps, the reduction in refrigerant ow can result

in a reduction of electric motor cooling leading to prema-ture motor failure.

Preventing cavitation requires keeping the pressure of

the liquid refrigerant above the saturation pressure cor-

responding to the refrigerants temperature as it enters

the pump. This is accomplished by ensuring the refriger-

ant being pumped has adequate net positive suction head

(NPSH).

What Is Cavitation?

expressed in gal/min. The vertical axis o the pump curve

shows the generated pump pressure dierential (head) ex-

pressed in psi.

The lines that project horizontally rom the vertical axis to

the right sloping downward represents this models pump per-

ormance with varying, but discrete, impeller diameters rang-

ing rom 8 in. (200 mm), the lowermost curve, to 10.375 in.

(264 mm), the uppermost curve. The dashed lines that run on

a diagonal rom upper let to lower right represent the required

pump power ranging rom 1.5 to 5 hp (1.12 to 3.73 kW). The

semicircular lines represent the pump efciency range rom50% to 65%.

Below the upper portion o the plot (i.e., the pump curve)

is an additional plot o the pumps NPSH requirement with

a 10.375 in. (264 mm) impeller diameter operating with

a discharge pressure o 23.7 psig (265 kPa) connected to

an ammonia recirculator operating at a pressure o 8.8 in.

Hg (40F [40C]). Since the system is operating in a

vacuum, we must frst determine the pressure developed by

the pump. The saturation pressure in the recirculator ves-

sel is 8.8 in. Hg (40F [40C]) or 4.3 psig; thereore,

the total head or pressure developed by the pump is 28 psi

(294 kPa). The ow delivered at that pressure dierential(i.e., head) can be determined by drawing a horizontal line

rom the 28 psi (294 kPa) hash mark to the 10.375 in. (264

mm) diameter impeller pump curve (blue line). The inter-

section o this pressure with the pump curve represents the

pumps operating point. Projecting a vertical line rom the

pump operating point down to the horizontal axis gives the

ow delivered by the pump. In this case, the ow is 140

gpm (31.8 m3/h). From this operating point, other operat-

ing characteristics o the pump can be obtained such as the

pump efciency (~64%) and operating power required (~4

hp [~3 kW]). The last piece o inormation is the NPSHr,

which is 2 t (0.61 m) in this case.


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Other Considerations

Minimum Flow Protection.While operating, liquid reriger-

ant pumps always need to move some amount o liquid to avoid

orming vapor within the pump due to heat addition rom the in-

efciency o the pump or motor (i its a semi-hermetic confgu-

ration). The added heat at low ow conditions can also cause the

liquid rerigerant in the pump to boil, leading to cavitation. To

prevent this orm o cavitation, a bypass (or minimum ow)

line rom the pump discharge back to the pumped recircula-

tor or recirculated liquid return is installed with an orifce (or

metering valve) set at a manuacturers required minimum ow.

During normal operation, the heat gain rom the pump powerinput is small, or the example we used earlier the heat gain

would result in a temperature rise o 0.18F (0.1C).

Presence of EPRs on Pump Discharge Pressure Require-

ment. Pumping liquid to evaporators equipped with evapora-

tor pressure regulators (EPRs) requires special attention. First,

the liquid eed pressure at the evaporator, and thus the pump

discharge pressure, must be higher than the EPR set pressure

to get liquid into the evaporator. Second, it is important to re-

alize that the pumped liquid rerigerant being supplied to an

EPR-ftted evaporator is subcooledas it enters the evaporator.

The liquid rerigerant entering the evaporator is essentially at

the temperature o the liquid in the pumped recirculator (plus a

slight temperature rise due to heat gain in the pump and in the

liquid piping). Since the liquid rerigerant supply temperature

is below the saturation temperature corresponding to the EPR

pressure setting, the liquid rerigerant entering the evaporator

has to absorb heat to sensibly raise the rerigerant temperature

until it reaches its saturation temperature corresponding to the

operating pressure prior to its boiling. Evaporators operating

with liquid rerigerant supply temperatures more than 10F

(5.6C) below saturation temperature will likely result in poor

evaporator perormance.

Static Head Requirements in Open Loops (Why VFDs

Are Not Great For Refrigerant Pumps).Another issue withopen loop pumping circuits is that the static head requirement

sets the minimum discharge pressure to get ow to the evapo-

rators. This means that beore there is any ow to the evapora-

tors, the pressure required to lit the liquid to the roo (or to the

elevation o evaporators) must be overcome. I the pressure

generated by the pump is not sufcient, the pump will operate

at its minimum ow with a column o liquid standing in the

riser supplying liquid rerigerant to the roo. In other words,

the pump is not capable o delivering ow to the evaporators.

This is the main reason that variable requency drives (VFDs)

are generally not suitable applications or rerigerant pumping

in liquid overeed applications. Note that a 50 t (15.24 m) rise

Figure 3: Pump curve for a liquid refrigerant pump.

gpm

10.375 in. 50

50

55

55

60

62 64 65

1.5 hp55

50

2 hp

64

6260

55

50

3 hp

8 in.

35

30

25

20

15

10

5

0

NPSHr

(ft)

Head

(psi)

40

30

20

10

0

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

5 hp

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210


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A u g us t 2 0 1 1 A SHR A E J our n a l 4 3

in an ammonia liquid line requires a pressure dierence o

approximately 15 psi (103 kPa) just to lit liquid to the roo.

Hydrostatic Lock-Up. Hydrostatic lock-up is the trapping

o subcooled (or pressurized) liquid rerigerant in a fxed vol-

ume and exposing that trapped volume to a heat source. As the

trapped liquid absorbs heat, it causes an increase in temperature,which causes the trapped liquid to volumetrically expand leading

to a substantial increase in rerigerant pressure and an increased

likelihood o component or equipment ailure. It is important

to identiy locations within a rerigeration system that can trap

liquid and provide suitable means o overpressure protection.

ASHRAE Standard 15-2010, Safety Standard for Refrigerating

Systems, provides requirements or protection rom hydrostatic

lock-up. The liquid eed valve train on an overed evaporator is a

location where hydrostatic lock-up is only a danger during main-

tenance procedures; thereore, can be eectively managed with

proper procedures and employee training. However, the liquid

supply piping between the pump discharge check valve and the

liquid eed solenoids on the evaporators must be protected rom

hydrostatic lock-up in the event o a plant-wide power ailure.

No amount o procedures or training can mitigate this hydraulic

lock-up scenario; thereore, the installation o a hydrostatic re-

lie device downstream o the pump discharge check valve piped

back to the recirculator vessel is required.

Summary

This article introduced concepts that are distinctive to

pumping rerigerants with centriugal pumps. In industrial

rerigeration systems, the use o centriugal pumps or sup-

plying low temperature liquid rerigerant to loads has be-

come quite common. Although a simple concept, the suc-cessul design, installation, and operation o centriugal

liquid rerigerant pumps does require care and attention to

a number o details. One o the most common operational

problems ought in the feld is rerigerant pump cavitation.

In a number o installations, we have ound the contributing

or root cause to cavitation is the excess ow o liquid through

the pump due to hand-expansion valves on individual evapo-

rators being set too ar open. When cavitation occurs, cool-

ing capacity is lost and the likelihood o premature pump

(or motor) ailure increases. We hope this article helps you

understand and troubleshoot potential problems that may ex-

ist in rerigerant pumping systems.

References

1. 2010 ASHRAE HandbookRefrigeration.

2. Stoecker, W.F. 1998.Industrial Refrigeration Handbook. McGraw

Hill Publishers.

3. Chaurette, J. 2011. Personal communication.

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