Refrigeration System

It is a natural phenomenon that 'heat' always flows from a body or space at a higher temperature to another body or space at a lower temperature.

Heat will not by itself, flow from a lower temperature level to a higher temperature level unless aided by an external agency.

Refrigeration is the process by which it is possible to remove heat from a region of lower temperature, transfer this heat to a higher temperature level by input of mechanical work and then reject this heat to a heatsink (such as ambient air or cooling water). The quantum of heat removed from the lower-temperature region is called refrigeration effect.

Concept by Analogy

The concept of refrigeration is better understood by an analogy with a simple hydraulic system:

Hydraulic System Transfer water, say from a ground-level tank, filled with water to an empty roof-level tank in a building, by using a bucket as 'water-carrier', in the following steps:

Step 1: Lower the water-carrier i.e. bucket below ground level tank and allow water from this tank to drain into and fill the bucket by gravity flow.

Step 2: Lift the bucket full of water from ground-level to a level higher than the roof-level tank by doing mechanical work.

Step 3: Empty the bucket by allowing its water to drain by natural gravity flow into the roof-level tank, empty bucket is now ready to resume water-lifting operation from Step 1 onwards.

Refrigeration System Transfer heat from a lower-temperature level body to higher-temperature level by using a heat-carrier substance known as 'refrigerant' in the following steps:

Step 1: Lower the heat-carrier;s, i.e.refrigerant's temperature below that of the low-temperature level body, to allow natural heat-flow from the lower-temperature body to the refrigerant.

Step 2: Raise the temperature level of the heat-carrier refrigerant now loaded with heat, to a temperature higher than that of heat-sink, by raising its pressure by doing mechanical work of compression.

Step 3: Reject the heat from the refrigerant, already raised to higher temperature level in Step 2, by natural heat-flow to the heat-sink (ambient air or cooling water). After draining its heat content to the heat-sink, the refrigerant is now ready to resume heat transfer operations, from Step 1 onwards.

Refrigerants

Refrigerants or heat-carriers in refrigeration systems are volatile liquids which change from liquid to vapour phase or vice-versa at different temperatures depending on the pressures these refrigerants are subjected to.

For example, water boils at 212 deg.F (100 deg. C) at normal atmospheric pressure, but when the water is subjected to higher pressures like in pressure-cookers, its boiling temperature becomes higher than 212 deg. F; whereas if water is subjected to pressure lower than atmospheric, its boiling temperature falls below 212 deg. F.

Different refrigerants have different pressure-boiling point characteristics. Refrigerants such as Ammonia (NH), halogenated hydrocarbons such as R-22 (CHCF) or even water-vapour are chosen with most desirable properties, keeping requirements of specific applications or systems in mind.

The Vapour Compression Refrigeration System and its Basic Cycle

Refrigeration Cycle

The majority of refrigeration systems for air-conditioning as well as process cooling applications use the vapour compression refrigeration cycle in which the heat added to the refrigerant at the lower temperature (from the space or body being cooled) is added as heat of vaporisation in the evaporator and the heat rejected by refrigerant vapour after being compressed and raised to the higher temperature level is rejected as heat of condensation in a condenser.

The Basic System and the Cycle are explained below for a R-22 refrigeration system often adopted for air-conditioning applications.

Refrigerant flows through the system components alternating between the liquid and vapour phases as indicated in the Basic Cycle.

Saturated liquid leaves the condenser at condensing pressure and temperature and is stored in the receiver to maintain steady supply of refrigerant for the system.

This high-pressure liquid refrigerant then enters the expansion valve, and undergoes the following processes:

1.Throttling 2. Evaporation 3. Compression 4. De-superheating and condensation.

Throttling (Expansion) Process

Expansion of the liquid refrigerant from the condenser pressure to the evaporator pressure is an adiabatic (Constant enthalpy) process.

Point 5 is located in the liquid-vapour mixture area of Pressure-Enthalphy since during this throttling process 'flash' vapour is produced by taking the required latent heat of evaporation from the liquid which is cooled down to a saturation temperature of 40deg. F at a reduced pressure of 83.72 psia.

Evaporation Process

Liquid refrigerant mixed with a small portion of flash gas, from outlet of the expansion valve at a reduced pressure of 83.70 psia, now enters the evaporator coil and fully evaporates at the corresponding saturated evaporation temperature of 40deg. F by taking the required latent heat of vaporisation, the air (flowing across the evaporator coil) being chilled in the process from 85deg. F to 55deg. F.

Compression Process

Saturated refrigerant vapour leaves the evaporator and enters the compressor suction port at point 6 of the cycle and its pressure and temperature is increased by compression until the superheated discharge gas at a pressure of 230 psia (i.e. same as condensing pressure) leaves the discharge port at point 9 of the cycle. Work done on the refrigerant gas during compression is known as heat of compression and is absorbed by the gas.

Desuperheating & Condensation Process.

This process takes place in the condenser. Intially the hot discharge gas is desuperheated by cooling down to the saturated condensing temperature corresponding to the condensing pressure of 230 psia and thereafter further removal of heat for condensing refrigerant fully to reach point 1 on the 100% saturated liquid line of Pressure-Enthalpy is effected by rejecting the heat of condensation' to the ambient air flowing over the condenser tubes.

Total heat rejected by the refrigeration system to the condenser cooling medium = heat absorbed by the refrigerant in the evaporator (refrigeration effect) + heat equivalent of work input during compression.

Ideal vs. Actual Vapour Compression Cycle

Ideal Vapour Compression Cycle

Schematic of Vapour Compression system.

An actual vapour compression refrigeration system differs in many respects from the theoretical cycle explained in Part 1, and shown above.

Heat transfer takes place between the refrigerant and the surrounding air in all components of the system as shown in the figure below; whereas the theoretical cycle considers only the "refrigeration effect" Qe (heat gained by the refrigerant in the evaporator from the conditioned space) and "heat rejection" Qc in the condenser to the ambient air, the actual cycle in practice must also account for the following additional heat gains and losses:

Qsl or heat gained by the already evaporated refrigerant in the suction line, from the ambient air.

Qcomp or the net heat transfer taking place from the refrigerant during the compression process to the surrounding air.

Schematic of Vapour Compression system.

Actual Vapour Compression Cycle

Hence the actual compression process deviates from "adiabatic" compression assumed in the case of the theoretical cycle because at the start of the compression process, heat is transferred to the cold suction gas from the warmer cylinder walls of the compressor. Towards the later part of the compression process, heat is transferred from the hot compressed refrigerant vapor to the cylinder walls and then to the surrounding air. Overall there is a net heat transfer from the compressed refrigerant to the surroundings.

Qdl is the heat loss from the discharge gas in the discharge pipeline to the surroundings. Qll is the heat loss from the hot condensed refrigerant in the liquid line to the surroundings.

Ideal vs. Actual Vapour Compression Cycle

Ideal Vapour Compression Cycle

Schematic of Vapour Compression system.

An actual vapour compression refrigeration system differs in many respects from the theoretical cycle explained in Part 1, and shown above.

Heat transfer takes place between the refrigerant and the surrounding air in all components of the system as shown in the figure below; whereas the theoretical cycle considers only the "refrigeration effect" Qe (heat gained by the refrigerant in the evaporator from the conditioned space) and "heat rejection" Qc in the condenser to the ambient air, the actual cycle in practice must also account for the following additional heat gains and losses:

Qsl or heat gained by the already evaporated refrigerant in the suction line, from the ambient air.

Qcomp or the net heat transfer taking place from the refrigerant during the compression process to the surrounding air.

Schematic of Vapour Compression system.

Actual Vapour Compression Cycle

Hence the actual compression process deviates from "adiabatic" compression assumed in the case of the theoretical cycle because at the start of the compression process, heat is transferred to the cold suction gas from the warmer cylinder walls of the compressor. Towards the later part of the compression process, heat is transferred from the hot compressed refrigerant vapor to the cylinder walls and then to the surrounding air. Overall there is a net heat transfer from the compressed refrigerant to the surroundings.

Qdl is the heat loss from the discharge gas in the discharge pipeline to the surroundings. Qll is the heat loss from the hot condensed refrigerant in the liquid line to the surroundings.

Pressure Drops in the Practical Cycle

Other variations take place in the practical vapour compression cycle due to the following system pressure drops which are inevitable in the "practical cycle'

Pressure drop of refrigerant in the evaporator coil, from the entering pressure condition of liquid-vapour flash gas mixture at the entry point to the evaporator till it reaches the state of fully evaporated refrigerant at the "exit" from the evaporator.

Pressure drop in the evaporator coil in the actual cycle is evident as the evaporation line 5b- 6b is slanting downwards instead of being horizontal.

Pressure drop in the hot gas line is from point 9a at the discharge from the compressor up to point 9b i.e. the entry point of hot gas to the condenser.

Pressure drop of refrigerant vapour in the suction line is from 6a, at the exit from the evaporator, to 6b i.e. point of entry to the suction port of the compressor.

The desuperheating-condensing-subcooling line from point 9b till it reaches point 1a is also shown slopping downwards on account of the pressure drop of refrigerant in the condenser.

The system designer therefore takes into consideration the pressure drops in the refrigerant suction, discharge and liquid lines as well as the estimated pressure drops in the evaporator and condenser in the practical system, while selecting equipment for the required performance at the expected actual suction pressure/temperature and discharge pressure/temperature conditions.

Also the refrigerant liquid, suction and discharge lines must be adequately sized for the required system capacity after considering location and layout of the refrigeration equipment so as not to exceed the permissible pressure-drop criteria in the refrigerant pipelines.

Two Stage Vapour Compression System

In the single stage vapour compression refrigeration cycle is explained. It will be observed that the hot gas entering the condenser is in a superheated state. The degree of

superheat depends on the "compression ratio" i.e. the ratio between the absolute discharge pressure and the absolute suction pressure.

For any given condensing temperature and pressure, which are determined mainly be the temperature of ambient air (in air-cooled systems) or temperature of entering cooling water (in water-cooled systems) as well as the heat-transfer area of the condenser, the "compression ratio" will depend on the suction pressure and temperature of the evaporating refrigerant, which in turn depends on the application the Refrigeration System either for air- conditioning (high evaporating temperature) or freezing application (low evaporating temperature)

In Refrigeration Systems applied to low temperature cold storage and freezing applications, the refrigerant temperature can be as low as 40 deg. F or lower, resulting in a very low suction pressure at the entry port of the compressor. Consequently the range through which the refrigerant gas has to be compressed to bring it to the condensing pressure condition becomes very large. In other words the " compression ratio" becomes very large for such low temperature cold storage or freezing applications even though the condensing temperature and pressure may be practically of the same order as in a moral air conditioning application.

In such high "compression ratio" systems the hot gas discharged by the compressor becomes highly superheated with very high discharge gas temperature. Since it is not desirable to operate the compressor with such a high discharge gas temperature, a "two-stage compression system" is adopted for such applications.

The first stage of compression compresses the suction gas to an intermediate pressure. The somewhat superheated discharge gas from the first-stage is usually cooled in an "intercooler" with the help of flash gas generated in the second-stage while expanding the high pressure liquid refrigerant from the condensing pressure to an intermediate pressure.

The mixing of discharge gas at intermediate pressure after the first-stage of compression with the cool flash gas from the second-stage of expansion will result in partial removal of superheat from the first stage discharge gas. Consequently and as evident from the cycle the discharge gas enthalpy at point 9 and corresponding discharge gas temperature after the second stage of compression in a two-stage system becomes less than the discharge gas enthalpy at point 10 and corresponding discharge gas temperature in a single-stage compression system, where the refrigerant is compressed in one step from suction pressure to final discharge pressure.

There is an added benefit of a two-stage refrigeration system, in as much as a greater part (approximately 62%) of the flash gas, which is formed during the second-stage of expansion, needs to be compressed through a smaller range i.e.. from interstage pressure to condensing pressure (instead of the entire quantity of flash gas requiring to be compressed through the total range of compression from "evaporator pressure" to the "condensing pressure" in the single-stage refrigeration in a two-stage refrigeration system) Thus the work input during the compressing process and the power consumption per ton of refrigeration in a two-stage refrigeration system becomes almost 15% less than for a single-stage refrigeration system.

Due to the benefits of better cycle efficiency and power saving in a two-stage refrigeration system with interstage cooling, the two-stage refrigeration system with interstage cooling the two-stage system is also adopted by some manufacturers for making Centrifugal chillers used in air conditioning applications even though the moderate "compression ratios" in chilled water air conditioning applications an be handled by a single-stage system. In fact manufacturers of single-stage Centrifugal machines tend to design their systems to operate at higher evaporating temperature and lower condensing temperature by somewhat oversizing the evaporator and condenser in order to offset the power disadvantage compared to two-stage Centrifugal systems.

The two-stage refrigeration cycle is explained by charting the sequence of flow of the refrigerant in different phases:

1-2: Refrigerant liquid leaving the condenser at condensing pressure flashes to "interstage pressure".

At 2: Refrigerant is partly gaseous (flash gas) and partly liquid.

6-7: First-stage compression process of refrigerant vapour from 'evaporator' pressure to 'interstage' pressure. Flash gas at point 2 at inter-stage pressure mixes with the first-stage discharge gas (point 7 ) to reach point 8, with lower superheat, before entering the second-stage compressor.

8-9 : Refrigerant vapour is compressed from inter-stage pressure to condensing pressure in the second stage of compression .

9-1: De-superheating of hot discharge gas and condensation to liquid phase in the condenser.

At 3 : Represents remaining portion of refrigerant in liquid at 'inter-stage' pressure, after removal of flash gas.

3-5: Liquid at inter-stage pressure flashes to evaporator pressure.

5-6: Evaporation of liquid refrigerant in the evaporator, before being drawn into the first-stage compressor.

6-7-10: If the entire compression process was carried out in one stretch, the compression line would have been 6-7-10 the point 10 representing the highly superheated condition of the discharge gas in a single-stage compression system.

Enthalpy difference between discharge vapour enthalpy at point 10 and the discharge vapor enthalpy at point 9 can be considered as the heat equivalent of power saved in a two-stage compression system compared to a single-stage system.

Cascade System

A cascade refrigeration system is but a variation of the two-stage compression system. The cascade system is often adopted for low temperature refrigeration and freezing applications.

There are two types of Cascade systems- one based on 'refrigerant-to-refrigerant' cascading and the other using 'chilled-water' As is evident in P-H and the schematic flow of a Cascade system refrigerant evaporating at a low pressure and temperature in the low stage evaporator is first compressed to an intermediate pressure in the low-stage compressor condensed in the shell side of a 'shell-and-tube' type heat exchanger, known as a 'cascade condenser' and then throttled in the low-stage expansion valve before being recycled through the low-stage evaporator.

The 'shell-and-tube' type exchanger referred to above as the 'cascade condenser' is generally a direct expansion chillier without baffles. Refrigerant from the high-stage system after throttling down to the intermediate pressure by the high-stage expansion valve is then circulated through the tubes of the cascade condenser and picks up its heat of evaporation from the heat rejected by the condensing refrigerant of the low-stage system (in the shell side) in the cascade condenser.

The high-stage refrigerant evaporating inside the tubes of the cascade condenser, is then compressed by a separate high-stage compressor from intermediate pressure to its condensing pressure. In this system, also known as 'refrigerant-to-refrigerant' cascading the refrigeration circuits for the two stages are entirely independent and even two different refrigerants can be used for the two stages.

In the case of 'chilled water' cascading system, chilled water produced by the high-stage refrigeration system is utilised as a condenser cooling medium for the low-stage

refrigeration system and is a relatively simpler system and has been successfully adopted in low temperature systems requiring not lower than - 40 deg. F temperature.

Vapour Absorption Refrigeration Systems Theory and Operating Cycles.

A vapour absorption refrigeration system is heat energy driven unlike the conventional vapour compression system which uses a compressor. Vapour absorption systems work with non-CFC refrigerants such as water or ammonia which evaporate at low-temperature in the "evaporator" maintained at low-pressure thus producing cooling at low temperature.

Absorption

Operation of the absorption cycle also requires a secondary fluid called "absorbent", (having great affinity for the refrigerant) which is used to absorb the gaseous refrigerant.

Figure 1

Figure 1. shows the basic flow diagram of the vapour absorption cycle. Instead of the low-pressure and temperature refrigerant being sucked into a compressor, (as happens in the vapour compression system) the refrigerant vapour is drawn into an adjoining "absorber " vessel containing the "absorbent" solution and gets readily absorbed into the

solution, due to its strong affinity for the refrigerant. The heat of condensation and the heat of mixing released during the absorption process is removed by another fluid such as "cooling water" or ambient air so as to maintain the low vapour pressure condition required for continuous evaporation of refrigerant and its onward absorption by absorbent solution to continue the cycle.

Generation

The absorbent solution which has become dilute after absorbing the refrigerant vapour in the absorber is then pumped by a solution pump to the higher temperature and pressure "generator"

In the generator the dilute solution is heated by means of steam/hot water or direct gas/oil firing so as to concentrate the solution and boil off and discharge the hot refrigerant vapour to the condenser.

The hot concentrated solution which has now regained its strong affinity for absorbing more refrigerant, then returns to the 'absorber' via a heat-exchanger after transferring its sensible heat to the cold dilute solution (being plumped from the absorber to the generator). This 'solution heat-exchanger ' improves the 'cycle efficiency' as preheating of the dilute solution in the heat exchanger before entering the generator reduces the heat-energy input to the generator and the simultaneous pre-cooling of the concentrated solution returning to the 'absorber' will reduce the extent of cooling to be done to the solution by the coolant, in the absorber section.

Characteristics and Limitations

Characteristics of Refrigerant- Absorption Pairs Chosen for Absorption Cooling

A fluid pair comprising lithium bromide salt solution as 'absorbent ' and water as refrigerant is commonly used for air-conditioning applications.

At the operating temperatures and pressures encountered, water as refrigerant is much more volatile compared to lithium bromide which is practically non volatile. Hence it is feasible to separate the refrigerant from the absorbent for proper evaporation of the refrigerant in the evaporator.

A concentrated lithium bromide solution has great affinity for water.

Since the operating pressures are low in the lithium bromide-water absorption system, the

pumping cost is low and also the wall thickness of the shell gets reduced compared to the earlier combination of ammonia-water fluid combination, which is now used mainly for low-temperature applications.

Limitations of Lithium Bromide-Water Absorption System.

Since water is used as refrigerant, evaporation temperature must be kept above the freezing point of water and hence the temperature of chilled water from such a system cannot be less than 5 deg C (41 deg. F)

Lithium bromide solution is corrosive. Therefore inhibitors need to be added to the system to protect the mental parts of the system against corrosion.

Coolant temperatures must be relatively lower to avoid crystallization of the lithium bromide which makes air cooling of the absorber, difficult in the lithium bromide system.

Ammonia-Water Absorption System

Figure 2

A fluid pair using ammonia as refrigerant and water as absorbent is now mainly used for low temperature refrigeration applications and to a limited extent for small capacity air-conditioning units with air-cooled absorption systems.

The advantages are:Water has great affinity for ammonia and can dissolve enormous amounts of ammonia vapour,and there is no solidification problem encountered with ammonia refrigerant over a very wide range of evaporating temperatures even up to very low temperature applications.

The limitations are: Since water also evaporates when the ammonia water solution is heated in the 'generator', the ammonia vapour from the 'generator' is mixed with water vapour. If the ammonia vapour mixed with water-vapour reaches the condenser, condensation of water vapour will interfere with the evaporation of the ammonia liquid in the evaporator of the ammonia liquid in the evaporator and reduce the refrigeration capacity.

Therefore the vapours from the generator are passed through the 'rectifier' where the vapour mixture is cooled so as to condense the water vapour and return the water to the generator and the rectified ammonia vapour then passes to the condenser, as shown in the

flow-diagram in Figure 3, to continue the remaining operations as in the case of the lithium bromide-water absorption system.

Single Effect Absorption Chiller

Single effect or single stage absorption cycle, as shown in Figure 1 uses "single-stage generation" for increasing concentration of the dilute solution by heating the solution in a single generator and boiling off the refrigerant vapour from the solution with heat input from low-pressure steam ( around 1 kg/cm 2 g pressure) or from hot water at ( 85 deg C to 95 deg. C)

Single effect system also has one "solution heat exchanger" for heat exchange between incoming cold dilute solution from the absorber and returning hot concentrated solution from the generator.

General Arrangement of Typical Single-Effect Absorption Chiller

figure 3

Figure 3 shows a schematic diagram of a single effect stream-fired absorption chiller, working on lithium bromide-water system, consisting of two sections in one shell. The lower section has two tube bundles -absorber tube-bundle at the bottom and the evaporator tube-bundle located above the absorber both operating at high vacuum conditions i.e. very low pressures of the order of 7 mm mercury absolute ( 1/100 atmosphere).

The upper section also contains two tube bundles the generator and the condenser operating at pressures of the order of 70 mm mercury absolute (i.e. 1/10 atmosphere) .

In addition there is a solution pump, a refrigerant pump, a heat exchanger, control valve and temperature controller and a purge unit (to remove non-condensable gases from the system).

Function of Main Components

Generator & Condenser Section

During operation, heat supplied by steam circulating inside the generator-tubes causes a portion of the refrigerant (water) to boil off, thus concentrating the dilute solution on the outside of the tubes.

The hot refrigerant vapour flows through eliminators (which prevent carry over of lithium bromide) to the condenser, where it is condensed on he outside of the tubes by giving up its heat of condensation cooling water passing through the tubes. This cooling water is the same water that has been previously used to cool the absorber.

Condensed refrigerant flows by gravity and pressure differential through a restrictor to the evaporator

Evaporator & Absorber Section

The liquid refrigerant (water) evaporates in the evaporator, at a low evaporation temperature of about 4 deg. C (corresponding to the low evaporator pressure of 7 mm mercury absolute) by taking the heat of evaporation from the return chilled water, say at 12 deg C (from the air conditioning system) which is cooled down to the desired outlet chilled water temperature, say 7 deg. C for supply to the air conditioning system.

Liquid refrigerant from the evaporator sump is pumped by a refrigerant pump and sprayed over the evaporator tubes through a spray header and nozzles, for improved heat transfer.

Refrigerant vapours from the evaporator flow through eliminators, (to prevent carry over of liquid refrigerant) to the absorber attracted and absorbed by the lithium bromide solution flowing over the outside of the absorber tubes, thus diluting the solution. Heat released during the process of absorption is removed by cooling water (from a cooling tower) flowing through the absorber tubes. The solution is circulated through the distribution system and sprayed over the absorber tubes by the solution pump for better heat transfer.

Heat Exchanger

The heat exchanger provided for heat exchange between the cold dilute solution before entering the generator and the hot concentrated solution leaving the generator, improves the 'cycle efficiency' by reducing he heat input required in the generator and reducing he cooling water flow in the absorber

. Capacity Control

Capacity of the unit is automatically controlled to meet the load variation, by varying the rate of re-concentration of the dilute solution in the generator by regulating the steam supply to the unit with a control valve at the steam condensate outlet getting a command for opening or closing from the 'sensor' sensing the outlet chilled water temperature.

Absorption Cycle Performance

Figure 4

The absorption cycle's performance can be read in Figure 4 which depicts the temperature pressure concentration relation for the lithium bromide aqueous solution in the entire cycle, as explained below

1. At Generator Outlet: shows concentrated and heated state of lithium bromide solution (65% lithium bromide by weight ) and temperature 103 deg. C and corresponding vapour pressure of 75 mm mercury Absolute.

2. At Absorber Inlet: Lithium bromide concentration remaining unchanged i.e. 65% but solution temperature coming down to 61 deg C (after passing through heat-exchanger) and vapour pressure 13 mm mercury Absolute.

3. At Absorber Outlet : Lower solution concentration of 60.3% reached after dilution and cold solution temperature of approx 38.5 deg C.

4.Inlet of Generator : Solution concentration remaining unchanged i.e. 60.3% (as in 3 above ) but higher solution temperature of approx. 78 deg C attained due to pre-heating in the heat-exchanger.

Coefficient of performance ( i.e. useful refrigeration effect in the evaporator divided by the heat input in generator ) of single effect absorption chillers is rather low (ranging between 0.68 to 0.75 )

However, a single-effect vapour absorption system offers the unique benefits of obtaining useful refrigeration for air-conditioning or process chilling through heat recovery from low-pressure and temperature waste steam in certain industrial processes or power plants using condensing back-pressure turbines, as well as heat recovery from a relatively lower-temperature flue-gas (such as a diesel engine/ gas engine exhaust ) to produce hot water for operating the single effect vapour absorption cycle.

HVAC DESIGN:

Central vs Floor-by-Floor AC Sytems

Author: Rita Kriplani, Senior General Manager(rtd), Engineering, Blue Star Ltd, Mumbai.Reproduced from Air Conditioning and Refrigeration Journal, Jan - Mar 1999.

The final choice of an HVAC system, whether central or floor-by-floor is a critical decision required to be taken before the facility design is completed.

Like modern office buildings, HVAC systems are available in a wide variety such as:Chilled waterDirect expansionPackaged and Split

The combinations of system and equipment variations, each with its own features, advantages and disadvantages are many.

Selecting the best system or combination of systems for a particular building must be carefully considered and researched by the consulting engineer in close coordination with the architect, electrical and plumbing consultants and owners, before finalizing the basic HVAC system design and building layout.

Detailed engineering, duct and pipe layouts, shaft locations and sizes, plant room dimensions etc, can follow in a systematic manner before construction work begins. This article discusses the various issues to be considered and the questions to be raised before an intelligent, well thought of HVAC scheme is finalized.

Some of the important issues in the Indian context are:

Cooling Medium - Air or Water

A decision on water or aircooled system will have an important bearing or plant room space requirements and whether enclosed or open.Availability of water is a basic aspect which determines the type of system, whether air cooled or water cooled.As water cooled systems consume much less power than air cooled systems and are more compact in size, it is advisable to go in for water cooled systems whenever water is available freely. The saving in lower could be approx 15% for reciprocating machines to about 30% for centrifugal machines,However, availability of water is not the only criteria for deciding on the water cooled option. The quality of water also plays a very important role. Water-caused scale formation problem is common in condensers of HVAC systems. Scale interferes with the transfer of heat in these heat exchangers. This interference is referred to as "fouling factor" in heat exchanger design terminology. A high fouling factor results in increase of compressor horsepower and loss of operating efficiency. Over a period of time scale formation occurs in HVAC equipment in varying degree and composition depending on the quality of water used. This calls for descaling of equipment periodically. Therefore it is important that quality of water in use is monitored and conrtrolled. Recommended water quality standards must be maintained for make up water, chilled water and condenser water. A water management system consisting of either a water softening plant or a reverse osmosis plant may have to be provided to control the quality of water used.In water cooled systems the chilled water circuit is normally a closed loop, while the condenser water circuit is an open loop, due to the use of a cooling tower. When using cooling towers, operating aspects like maintaining water level in the cooling tower basin, regulating the bleed-off and controlling make up water quality, call for regular checks. It is also important to keep the system clean of algae and bacteria.

The air cooled system, on the other hand, consumes 20% to 25% more power.However, the heat of the system is rejected to air which is available in abundance. There are no water shortages to contend with, no make up water tanks that can run empty, no condenser pumps, no condenser water piping, no condenser descaling.

On the whole, air cooled systems are much easier coils can be easily cleaned, periodically, on the air side.

Architectural Features and Space for HVAC

The architectural design of the building is an important aspect that must be studied before finalising the system selection. The following aspects of the building design can help the engineer to determine the right selection:

Plant room space in basement with adequate height of 4.3 to 4.9 metres for water cooled systems, or open space on terrace or ground level for air cooled chillers.Clear height available above false ceiling for running ductwork.Space available for installing AHUS near shafts.Accessible space above false ceiling of ac area or passages for mounting split unit coolers and accessible space for condensing units.

Possibility of locating fresh air intakes on building facia above false ceiling level to facilitate fresh air intakes for split units.

Space for shafts to carry chilled water and condenser water pipes.

Availability of drain lines in peripheral area or core area to facilitate drainage from split unit coolers.

Floor-by-Floor System, water cooled

The water cooled, floor standing packaged AC is the type that can be most conveniently located in a separate room adjoining a shaft in the core of a new building. The shaft houses the condenser water piping connected to a common building cooling tower on the terrace. The largest packaged unit available in India today is 15 tons which can handle an area of approx 300 sq.m. A larger floor area, can be handled with multiple units as long as the building design can accomodate additional shafts close to the units for the condenser lines. Multiple units can also help reduce the supply air duct sizes and thus increase the false ceiling height which all architects and clients just love. Aesthetically, the higher the false celling height, within economic limits of course, the better the acceptability of an office, Low heights increase the feeling of claustrophobia among office staff.

Floor-by-Floor System, air cooled

Where water availability is a problem, air cooled splits can be used. However, the building design must accommodate:

A two feet high indoor cooling unit with proper access for maintenance.The unit can be ceiling suspended or sitting on a loft.The floor-by-floor building height must be adequate for mounting such a unit and still leave enough space below for a decent false ceiling height. A drain line with proper slope is a must from each unit to a floor drain.An outdoor condensing unit with safe and proper space for maintenance.This unit should be not more than 20 metres horizontally from the indoor unit and vertically not more than 10 metres higher than the indoor unit. Shorter distances hellp improve cooling capacity and longer distances reduce capacity.

The largest single piece air cooled split unit available in India today is 8.5 tons capable of cooling 160 sq,m approx. Larger floor areas can use multiple units which can also help in reducing duct sizes.

Existing buildings with a low floor-to-floor height can presient proboems with equipment layout. Very often such buildings have heights of only 2.7 m or less with deep criss crossing beams leaving clear heights of only 2.3 or 2.4m. Mounting ceiling suspended indoor cooling units in such building leaves one with a clear height of only 1.8 m or so making the space below unuseable for an office. A filing room or a storage room at best is what is available. The outdoor units are mounted on the overhanging sun shades or chajjas or on steel platforms bolted to the outside building walls-making maintenace extremely risky and difficult. Buildings also start looking shabbly and disfigured.

Fresh air

Introducing fresh air into offices is extremely important for user health and better indoor quality. It is relatively simple to provide fork fresh air intakes when designing a new huilding having packaged or split units or providing a separate treated fresh air unit for the entire floor. However, in an existing building such openigns for fresh air unit are often difficult and hence conveniently forgotten with poor results on occupant confort and health.

Many engineers design systems with split ACS having indoor cooling units mounted above toilets, as this is the only space available, and then install a false ceiling below with an access door for maintenance.Such locations for cooling units are improper as toilet odors leak into the return air path which is under slight negative pressure and leads to complaints of mal-odour from users.

Central Plant

A central plant, will require plant room space on the ground floor or in the basement to house water chilling machines, condenser water pumps, chilled water pumps, condenser/chilled water piping and electric panels. The plant room size will depend on the size of the plant. These plant rooms require a minimum clear height of 4.3 to 4.9 m to accomodate equipment, pipe headers and cable trays. If the system has ice/chilled water thermal storage, huge tanks are required to be accommodated. A direct fired absorption system will require fuel storage tanks and space to accomodate fuel handling equipment. A steam fired absorption machine will require space to accommodate the boiler.In addition, each floor has to house the air handling system consisting of fan section, cooling/heating coil section, filter sections and electrical panels. These AHU room sizes can vary from a room of 3x3m to 6x 4 m depending on the size of the plant.

A shaft is needed to house chilled water piping, condenser water piping (if the cooling tower is on the terrace) and power/control cables. Each AHU room must be provided with drainage and fresh air intake.

The air handling capacity of the AHU which will depend on the floor area and the cooling load to be handled, will determine the duct size leaving the fan outlet. If this size is too large to permit a reasonable false ceiling height it may be desirable to consider two smaller. AHUs on each floor, provided, a second shaft can be fitted into the floor layout.

If an air cooled central plant is envisaged, adequate space is required on the terrace to place the chilling machines and chilled water pumps. The electrical panel should preferably have IP55 rating. Normally space is also allocated for standby chilling machines and pumps. Consideration must be given to the noise of the chillers and whether this will affect adjacent buildings.

Thought must be given to the access path to plant rooms and AHU rooms. In case of a breakdown, the machine may have to a be shifted to a service shop for repair. The building design must provide this space. The structure should be designed to take the weight of equipment in position and along this access path. Adequate load bearing beams and columns must be available for lifting and shifting of such equipment.If the plant room is in the basement adequate drainage facilities are a must, as the water in the system may have to be drained in case of a major shutdown. In multistorey buildings this water volume can be very large, creating a serious drainage problem. Isolating valves are normally recommended to avoid such problems.

Owner's Needs

If the architect is the creator, the owner is the King and his needs and requirements must be met. So talk to him and find out his needs before proceeding with the design.The user profile of multi-storey office buildings can vary to a great extent.The complete building may have either a single owner or multiple owners.A single owner normally has a preference for a central plant as the quality of air conditioning is far superior. In additioon the owners can opt for an intelligent building by incorporating a

Building Management System. This will enable the owner to derive benefits of optimal utilisatioon of the air conditioning plant.

A multiple owner facility requires a system which provides individual energy billing for which a floor-by-floor air conditioning system using packaged units or split units is most suited.

Another important requirement is the normal working hours of the user/users. Some users may have different working hours or different timings. Some areas such as computer rooms may need 24 hour air conditioning. Other areas may have special design requirements.

Due to such multiple requirements many engineers prefer a combination of a central plant and packaged units/split units. Such systems offer high flexibility in meeting the requirement of different working hours and special design conditions.

Standby or Redundancy

Even a Rolls Royce engine on an aircraft can suffer a breakdown! But how serious is the repercussion and how often does it occur? Some clients want comfort irrespective of breakdowns and for them standby equipment is a must.

An office complex generally requires a standby cooling machine to ensure that air conditioning is always available. A central plant system cans easily accommodate a standby packaged chiller with pumps in the same plant room. These units are connected to common condenser water/chilled water headers thus minimizing the requirement of additional space. Air handling units are normally not provided as standby, as the breakdown rates are insignificant. A few standby motors can be kept as spares in the premises for such units. In a floor-by-floor air conditioning system using packaged units and splits it is not always possible to provide a non-working standby unit. Normally such units are installed in multiple and are distributed over the air conditioned space. Therefore whenever a unit suffers a breakdown, air conditioning is inadequate causing user complaints.

Initial Cost

Everyone cares about cost! But the wise knower lays down a list of minimum requirements and then negotiates. The "penny-wise pound-foolish" owner goes for price only and skimps on equipment and design specifications.The initial cost of a central air conditioning system is much higher than a floor-by-floor system. Depending on the type of equipment selected the cost can vary to a great extent. For example, a reciprocating packaged chiller is much cheaper than a screw packaged chiller and the screw packaged chiller is cheaper than a centrifugal chiller. Air cooled machines are costlier than water cooled machines. Therefore, the budget available with

the owner at the time of building the facility play a major role in selecting the type air conditioning system.

When it comes to air handling units, the single skin air handling unit is much cheaper than a double skin air handler. However, the system cleanliness and aesthetics achieved by using double skin air handling units is far superior. The life expectancy of these units is also higher.

VAVs and a building management system if added, will increase the capital cost by 10%-15%. However there will be a saving in power cost and so it is important to work out the payback period to determine the techno-commercial liabilities of the final selected system.

Engineering Cost

Whenever a major facility like a multi-storey building project is designed, it is imperative that an HVAC engineer be involved from the initial stage itself. Such a design and build approach will lead to a well coordinated effort between the architect, HVAC engineer, builder and client. Such involvement will also provide expertise to evaluate and analyze the techno-economic aspects of each system. The system selection must precede the final architectural design of the building. Even though such engineering inputs seem to add to the cost and time of the project.

Engineering cost, time and risk factors for designing a unitary floor-by-floor system are usually lower than those for a central system for the following reasons:Load calculations and corresponding equipment selections are less critical with packaged floor-by-floor systems. The multiple numbers of modular units will provide built in safety cum flexibility into the design.Unitary or packaged systems are factory built standard equipment. The quantum of work to be carried out at site is much less as compared to central plant system as the amount of ducting piping and insulation work is much less.Engineering skill, cost and time required to install a floor-by-floor packaged system is much less as compared to a central plant. A central plant design envisages equipment layouts, ducting layouts, piping layouts, which are much more complex. Layout finalization is also time consuming as these designs are required to be well integrated with structural, interior layouts and other utilities. Floor-by-floor system layouts are much simpler and repeated multiple times.

Installation Cost

The mechanical installation cost of a central plant is normally much higher than a floor-by-floor system due to the following reasons.Main air conditioning equipment is heavy and voluminous requiring strong foundations and proper material handling facility at site.

Air handling units/cooling towers/fans must be lifted to upper floors or terrace.Some equipment requires extra care during installation to ensure mininum vibrations and smooth operation.Larger quantities of ducting, piping and insulation are required and their installation cost is higher.

O and M Cost

A low initial cost plant can end up being an energy guzzler. With energy costs rising by the year and already at a high level it pays to determine your "operating and maintenance costs" before its too late.In a centralized non-thermal-storage chilled water supply system the chillers normally operate at less than full capacity and adjust that capacity to follow the building load changes. In other word chillers operate at off design conditions, most of the time. Off design conditions are any operating parameters that differ from design selection point of the chillers. A part load operation is a part of off design condition but it is not the entire set of conditions. For all AC systems a vast majority of operating hours are spent at off design conditions.Therefore it is important select machines which the best off design performance.

Recently the Air-conditioning and Refrigeration Institute (ARI) has replaced the previous Integrated Part Load Value/Application Part Load Value (IPLV/APLV) rating. The new rating closely tracks real-world chiller energy performance by more accurately accounting for chiller operation at off-design conditions. The new rating termed IPLV/NPLV for Integrated Part Load Value is a part of new standard has become effective from December 1998. This change by ARI recognise that chillers rarely operate at design conditions, because design contitions mean the simultaneous occurrence of both design load and design.

Entering Condenser Water Temperature (ECWT) or design Entering Dry Bulb (EDB) temperature.It is observed that the design ECWT or EDB occur during less than 1% of chiller oerating hours. This means that over 99% of potential chiller operating hours are spent at off-design conditions. In India where high temperatures are fairly constant over a large period of time these percentages may vary slightly. Engineers can now eliminate the specifications of design kW/ton which is merely an efficiency at one condition and provides no indication of off design performance. In fact chillers with the best design lW/ton may have the worst IPLV/NPLV performance because they were optimized for design conditions. Therefore IPLV/NPLV ratings should be used to ascertain the best chiller for a given applicaton. The new ratings track the performance of multiple chiller plants, as well as single chiller plants.

The modern centrifugal machine is capable of operating at a power consumption of 0.50 - 0.60 kW per ton.In addition to the above, centrifugal machines are now available with variable speed

drives (VSD) Electronic VSD technology enables machines to operate at off design conditions at 0.40, 0.30 and even at 0.20 kW/ton This leads to an unprecedented energy saving.Annual energy costs of a chiller can be easily projected using this formula:Annual energy cost = NPLV * Rs/ kWh = Average chiller load x operating hours.The operating cost of the system finally depends on over-all system efficiency. The cost of power in a place like Mumbai can be as high as Rs 6 per unit for commercial buildings. The design engineer, therefore, must ensure that all auxillary equipment like condenser water pumps, chilled water pump, tower fans are included when considering efficiency of the system.On the low side of the central AC system, air handling units are the biggest consumer of power next to the chillers. Normally, constant volume air handling units are provided, which consume the same energy day in and day out irrespective of variation in load. By incorporating VAVs with variable speed drive on air handling units it is possible to achieve excellent savings in power. Saving in power could be as high as 30% -50%.

Floor-by-Floor System

The power consumption of water cooled packaged machines can vary from 1.0 kW per ton to 1.2 kW per ton and the power consumption of air cooled splits varies from 1.3 kW to 1.6 kW per ton. The type of compressors used in these machines are either hermetic reciprocating or scroll. The part load efficiency of such units is lower than their full load efficiency.

Maintenance Cost

The breakdown and maintenance cost of central plants is much higher as the system has several large and expensive items. However, the frequency of such breakdown is quite low in a central plant. Repair/replacement of equipment due to breakdown or total failure can be expensive and time consuming. Additionally, these systems require routine inspection and planned checks. Daily operation also adds to the running cost as trained operators are required.The floor-by-floor system repair cost per break-down is normally low. With the emergence of reliable hermetic and scroll compressors, their maintenance expenditure has shown remarkable an improvement is less time consuming and simple.

A typical example of the operation cost incurred at a central plant installation for Commerce Centre Cuffe Parade and a floor-by-floor AC plant installation at Metropolitan, Bandra Kurla Complex.

Commerce Center, Cuffe Parade. Mumbai

Number of Storeys Basement, ground  + 32 floors

Air conditioned area 18,089 sqm (4,10,000 sq.ft)

User profile Multi users

AC system:Central plant

4 nos R-11 centrifugal chillers total capacity 2200 Tr34 air handling units4 chilled water pumps4 condenser water pumps3 cooling towers

Cost of the AC plantwith 0.9 kW/ton)

Rs. 54,500 per tonRs. 220 per sq.ft.

Air conditioned area 18,089 sqm (4,10,000 sq.ft)

Annual power consumed  Rs. 12.81 per sq.ft.

Metropolitan-Bandra Kurla Complex, Mumbai

Number of Storeys Basement, ground  + 9 floors

Air conditioned area 5,202 sqm (56,000 sq.ft)

User profile Multi users

AC system:Air cooled split units

50 nos 7.5 Tr split units. 375 Tr

Cost of the AC plantRs. 30,000 per tonRs. 200 per sq.ft.

Annual power consumed  Rs. 78.50 per sq.ft.

Level of Comfort

At one time most people were satisfied with an air conditioning system if it maintained low temperatures. Today users want better and individual temperature control, lower humidity, cleaner air and no air drafts.The quality of air conditioning and the control on the design parameters is much superior in central AC system. Such systems are normally provided with higher efficiency filters and it can handle the required quantity of fresh air with an in-built capacity of absorbing latent load.This result in a high relative humidity at full load as well as part load.Such variations in peripheral and core air conditioning load can very well be handled by a central plant with a VAV system. The normal practice in India is to provide a constant volume system whose design is based on the sum of the peak loads, which results in over cooling during off design conditions. Individual comfort conditions can be achieved by providing heaters in branches, but this results in high energy wastage. As the peak time depends on wall orientation, the sum of the peaks is always higher than the instantaneous block load. Therefore it is advisable to have a variable air volume system whenever individual comfort conditions are important. VAV system design is based on block load calculations, as the VAV units allow the system to borrow air from areas with low load. By incorporating VAVs with variable speed drive on air handling units, it is possible to achieve excellent savings in power, which can be as high as 30 - 50% Even though the inital cost of the plant increases by 7% - 10% due to VAVs and variable speed drives, the pay back is normally less than 2 years.

Indoor Air Quality

Many office workers complain of frequent colds, headaches and unclear thinking as the day drags on. Most of them don't realise that such problems are caused by 'Sick' buildings with inadequate fresh air and inefficient air filters. That's what indoor air quality or IAQ is about!It has now been recognized that it is important to maintain an acceptable indoor air quality to safeguard the health of occupant ASHRAE Standard 62-1989 on "Ventilation for Acceptable Indoor Air Quality" recommends a minimum standard ventilation rate of 15 cfm per person in office areas. This represents an increase from the previous ASHRAE minimum standard ventilation rate of 5 cfm per person.Indoor air quality is considered acceptable if the required rates of outdoor air are provided for the occupied space. As human occupants produce carbon dioxide, water vapour, particulates, biological aerosols, etc,. the carbon dioxide concentration has been accepted as an indicator of indoor air quality. Comfort (odor) criteria are likely to be

satisfied if the ventilation rate is set so that 1000 ppm carbon dioxide is not exceeded.It is possible to control indoor air quality in a central plant by designing the main air handling system to cater for the required outdoor air treatment. Further it is possible to incorporate strategies which are desirable with increased ventilation rates:

Increased recirculation with high efficiency filters.Heat recovery devicesAutomatic carbon dioxide monitoring for improved control.Improved air distribution.

Combined variable air volume technology and automatic CO2 control enables a system that already responds dynamically to temperature and humidity to also respond dynamically to indoor air pollutants.

From the air quality perspective, infiltration only occurs at the bottom and top few floors. In the center of the building infiltration effects are minimal, and therefore it is advisable to have a well sealed building and control the air distribution.

In a floor-by-floor unitary system the common practice is to provide fresh air openings near the equipment. However, to maintain acceptable indoor air quality it would be advisable to install a separate air unit which can supply treated fresh air to each packaged/split unit.

SUMMARY:

Central Plants offer:

Offer Greater Variety of EquipmentTechnology and selection range available is far greater than floor-by-floor unitary systems. The following equipment both indigenous as well as imported, is commonly used in India:

Centrifugal chillers 150-1300 Tr

Water cooled screw chillers 100-400 Tr

Air cooled screw chillers 100-300 Tr

Water cooled reciprocating chillers 30-200 Tr

Air cooled reciprocating chillers 10-200 Tr

Steam fired absorption chillers 150-1500 Tr

.Direct fired absorption chillers 300-1500 Tr

On the other hand the variety and capacity range of packaged units and split units manufactured in India is limited. Such units are supplied with hermetic compressors without any capacity unloading. Maximum capacity available is 15 tons. In countries like the USA, packaged units with semi-hermetic compressors are available up to 100 ton capacity with multiple compressors.

Have Better Part Load PerformanceCentrifugal and screw chillers give better part load and off-design performance than packaged and split units.They also offer turn down ratios upto about 20% by employing capacity control methods like VSD for centrifugal chillers and modulating/sTepped slide valve control for screw chillers. Absorption chillers also offer good part load performance and economical turndown upto about 25%. Semi hermetic and open type reciprocating chillers have stepped capacity controls, however, the part load efficiency of a reciprocating machine is lower than its full load efficiency.

Are better suited to BMS

BMS enables the owner of a central plant to have:Automatic operationOptimized equalizationLoad Rolling/SequencingFinger tip information on Operation parametersEnergy usageMaintenance recordsBreakdown historyRunning cost controlSystem interface with other utilitiesA BMS is considered a strategic investment as it can result in life long savings.On the other hand, floor-by-floor unitary systems have limited scope for BMS application.

Have a longer lifeA well maintained central plant has a life expectancy of about 20 to 25 years. Capacity control methods like variable sped drives increase life of chillers and air handling units. A double skin AHU has much longer life than a single skin AHU. Packaged or split units have a life expectancy of about 12 to 15 years.

Easier to Provide for RedundancyIn the central plant system it is easy to provide for redundancy by installing a standby chiller and pump in the same plant room.A multiple chiller plant with a standby not only provides redundancy at full load, but it also provides for more than 100% redundancy from the operating chiller. This is because of the fact that most of the chiller plant operate at off-design conditions for 99% of the time. By allowing individual chillers in a multiple chiller plant to work at higher loads additional standby facility is normally made available.

Can Provide Better Indoor Air QualityThe cooling coils in a central pllant can be specially designed to handle higher latent loads and thus provide better indoor air quality by increasing the quantity of fresh air.Multi stage filtration can improve the quality of supply air and the fan static pressure can be selected to suit the pressure drop.On the other hand, in floor-by-floor systems, it is not possible to provide a high level of filtration or increase the fresh air quantity.

Are Preferred By Prestigious OwnersMost prestigious buildings with a single corporate or Government owner, prefer to install central plants as the quality of air conditioning is superior and life expectancy is higher. The operation and maintenance cost is lower than a floor-by-floor system.

Floor-by-floor packaged systems have advantages of:

Lower First CostPackaged and split units have much lower first cost than a central system. However, the life expectancy of floor-by-floor system is much lower at about 12 to 15 years only.Faster InstallationEasy to instal and less time consuming than a central plant. Since standard size units are readily available, replacements can be carried out very fast.

Individual OwnershipEach tenant can own his air conditioning plant, operate it at his convenience and pay the individual power bills. Therefore, when a building complex has a multiple owner profile,l a floor-by-floor system is preferred.

Conclusion

The final choice of an HVAC system, whether central or floor-by-floor is a critical decision required to be taken before the facility design is completed. The team consisting of owner, architect and HVAC design engineer need to integrate the users requirements with the architect's vision, carry out a techno- economic evaluation of various types of systems after scrutinizing all aspects explained in this article. The finally selected system must fit in to the owner's capital budget and anticipated life cycle operation and maintenance cost. There is a growing trend to select a combination of central plant and packaged or split units to meet the requirements of the user and for his complete satisfaction.