Cooling

To warm air or water energy in the form of heat must be added to it. The converse is also true; to reduce the temperature of air or water energy must be removed from it. The system which is used by the majority of air conditioning systems is based on the vapor compression cycle. A less common system is absorption chilling.

It should be noted that the term "cooling" usually relates to the direct production of cold air whereas the term "chilling" relates to the production of cold water. This cold water is then circulated through the cooling coil of an air handling or fan coil unit to cool the airflow.

Vapour compression cycle. Most people have daily contact with cooling caused by the vapour compression cycle in the form of the domestic refrigerator. The refrigerator is a useful example to keep in mind whilst considering how the system works. Cooling systems used in buildings use the same principle but on a different scale. Below figure shows the components of a vapour compression chiller.

The main components are an evaporator coil, a compressor, a condenser coil, and an expansion device. These components are connected together using copper pipe through which refrigerant circulates in a closed loop. Cooling is achieved in the following way;

Liquid refrigerant is forced through the expansion valve. As the refrigerant leaves the expansion valve its pressure is reduced. This allows it to evaporate at a low temperature. For any liquid to evaporate it must absorb energy. The refrigerant evaporates by removing energy from the evaporator coil which in turn removes heat from the air which is flowing over it. Hence the air becomes cooled. The refrigerant, now in a vapour state, leaves the evaporator and passes through the compressor. The pressure is increased causing the refrigerant vapour to condense in the condenser coil. This occurs at a relatively high temperature. As the refrigerant condenses it releases the heat it absorbed during evaporation. This heats up the condenser coil. Air passing over the condenser coil takes away this waste heat.

In terms of a domestic refrigerator the evaporator would be situated in the ice compartment and the condenser is the grid of piping at the rear of the refrigerator which is warm to the touch. In building cooling systems significant amounts of waste heat are produced at the condenser and various techniques are used to safely remove it from the building. The method of heat rejection depends on the amount of waste heat produced and operational decisions such as the choice between using a dry system or a wet system.

The evaporator and condenser coils are simply arrays of copper pipe with aluminum fins mechanically bonded to their surface to increase the area for heat transfer.

Refrigerants and Compressors

The following articles will consider refrigerants and compressors which are two of the more complex components of vapour compression chillers.

Refrigerants. Refrigerants are liquids that evaporate very easily at relatively low temperatures. Refrigerants are so volatile that if a liquid refrigerant was spilled in a room at normal temperatures it would very quickly disappear by evaporation. Refrigerants must posses good thermodynamic properties but also have low toxicity and low flammability. Refrigerants are also the subject of environmental concerns as described (CAREFUL USE OF REFRIGERANTS).The evaporation and condensation of refrigerants in a chiller is controlled by lowering and increasing the pressure using the expansion valve and compressor respectively.

Compressors are electrically driven pumps of which there are three main types. These are; reciprocating, rotary and centrifugal compressors.

Reciprocating compressors work by allowing refrigerant

to flow into a chamber on the down stroke of a piston.

The refrigerant is then forced out of this chamber towards

the condenser as the piston moves upwards once more. 

Rotary compressors have two interlocked helical screws 

which when rotated move refrigerant which is trapped between 

the two screws along the line of the thread. 

Centrifugal compressors have a rotating impeller which 

forces the refrigerant outwards against the casing. 

This force is sufficiently strong to drive the refrigerant 

towards the condenser. 

Compressors are further classified as either hermetic, semi hermetic or open depending on the seals between the motor and the compressor it drives. Hermetically sealed compressors have the motor and compressor together inside a shell whose seams are sealed by welded joints. Refrigerant is in contact with the motor and compressor. Semi hermetic compressors are similar but the joints are bolted rather than welded allowing servicing to take place. Open compressors have an external motor connected via a shaft to the pumping mechanism. A seal around the shaft stops refrigerant escaping. The different forms of compressor are suitable for different cooling load ranges. Reciprocating up to 180kW, Rotary up to 2MW and centrifugal in the range 180kW to 3.5MW. Comfort cooling tends to be of a lower cooling capacity and so uses reciprocating compressors. Rotary and centrifugal compressors are used for large capacity centralized cooling systems.

Careful Use Of Refrigerants

Some refrigerant gasses are known to damage the ozone layer. 

However, they can only do this if they are allowed to escape from the system.

It is estimated that 20% of refrigerant based systems will develop a leak resulting in a complete loss of refrigerant charge during their operational lifetime. Depending on the type of refrigerant the risks from this vary from fire and toxicity to global warming and ozone depletion. There are a number of methods of minimizing or avoiding these problems. These include reducing the volume of refrigerant in the equipment, developing benign refrigerants, good practice including servicing, designing systems to avoid leaks and detection and rectification of leaks.

REDUCTIONS IN REFRIGERANT CHARGE

If a leak occurs the amount of damage caused depends on the volume of refrigerant that has escaped. It follows therefore that if the volume of refrigerant used to charge the system can be reduced the effects of a total leak will be minimized. This can be achieved in a number of ways. The first method is to reduce dependence on refrigeration equipment. This can be achieved using passive cooling techniques. This may eliminate the need for refrigerants completely or reduce the size of the system required. The second involves choosing equipment that has a high efficiency. This means more cooling can be carried out with fewer refrigerants. Hydronic systems can be used where the chilled water is created in a local plant room. This is then used in the building rather than use refrigerant pipework through the building which would increase the number of refrigerant components through which a leak could occur.

LEAK DETECTION

Refrigerant leak detection takes two forms, visual and gas analysis. Visual systems require a fluorescent dye to be added to the refrigerant. If the refrigerant begins to leak out of the system say through a loose joint then this will be revealed under an ultra violet light as a glowing patch of dye. In this way the exact location of the leak can be pin pointed. Unfortunately the leak can only be detected if the system is inspected regularly. The second type of leak detection involves drawing a sample of air surrounding the refrigeration equipment into a gas analyzer. The analyzer will detect and warn of the presence of refrigerant in the air sample indicating that a leak was occurring. This system can be set up to continuously monitor a plant room for the signs of a leak. Pin pointing the leak would require a further inspection of the system using either a hand held detector or searching for the presence of escaped dyes as described above.

RECYCLING OF REFRIGERANTS

The production of a number of refrigerants has been banned under the Montreal Protocol and EU regulations. However existing stocks can still be used to service older equipment. It follows therefore that the refrigerant contained in systems about to be replaced has considerable value to existing users. It is illegal under the environmental protection act to release substances into the environment which are known to cause damage. Because of this all refrigerant should be removed from the system and stored before repair or decommissioning. If the recovered refrigerant is of good quality it can be re used without further treatment. If the refrigerant is contaminated with oils, acids, moisture or particles then the refrigerant must be cleaned by filtration and distillation before being re-used. Heavily contaminated refrigerants must be reclaimed this requires that they are taken off site and purified to their original state.

A good network of refrigerant reclaimers and recyclers is important to manage refrigerants and deter the growing trade of smuggling illegal refrigerants into the country.

Refrigerants and the Environment

Some refrigerants have been identified as contributing to ozone depletion and global warming

During the last decade some refrigerants have been identified as ozone depleting gases and/or greenhouse gases. As a consequence the chemical companies producing refrigerants have been working to find alternative refrigerants which have a good blend of physical and thermodynamic characteristics but do not damage the environment if they escape. At the same time governments have brought in legislation which bans the production and use of the more damaging refrigerants. The most well-known of these pieces of legislation is the Montreal Protocol on substances which deplete the ozone layer. This legislation has banned the production of the most ozone depleting refrigerants and has set a time limit on the manufacture of less damaging refrigerants. Many governments and the EU have brought in more strict legislation shortening timescales meaning that bans are now in place.

Refrigerants are identified in the building services industry by a refrigerant number. For example, R11 and R12 are chlorofluorocarbons (CFC's) which are highly destructive to the ozone layer and their production is now banned. R22 is a hydrochlorofluorocarbon (HCFC) which is less damaging to the ozone layer than CFC's and so its production is allowed until 2005. Existing stockpiles of both refrigerants can still be used. R134a is a Hydrofluorocarbon (HFC) it contains no chlorine and so does not damage the ozone layer. However like other refrigerants it is a global warming gas.

There are three indices that are used for comparing the environmental effects of refrigerants:

Ozone Destruction Potential (ODP) - A measure of how destructive the chemical is to the ozone layer in comparison to R11 which is said to have an ODP = 1

Atmospheric Lifetime - The length of time, measured in years, that the refrigerant remains in the atmosphere causing ozone destruction.

Global Warming Potential (GWP) - A measure of the contribution the chemical makes to global warming in comparison to CO2 whose GWP = 1.0.

The table below compares these indices for various refrigerants. It can be seen that R134a has a zero ODP but still has a global warming potential. Environmental groups are now campaigning against HFCs because of their GWP. However, the dominant factor in global warming is CO2 emitted (from power stations) as a result of electrical consumption by the chiller rather than the global warming effect of escaped refrigerants. A method of quantifying the contributions from each is given by the Total Equivalent Warming Impact (TEWI). This is a lifecycle analysis which considers both the direct global warming impact of the escaped refrigerant and the efficiency of the refrigeration system as a whole.

Refrigerant	Type	ODP	Lifetime	GWP
R11	CFC	1.0	60 years	1500
R22	HCFC	0.05	15 years	510
R134a	HFC	0.0	16 years	420
R290	Propane	0.0	<1year	3
R717	Ammonia	0.0	<1year	0
Lithium Bromide		0.0	<1year	0

                          *Environmental Indices Table

Environmental concerns have also led to renewed interest in traditional refrigerants such as ammonia and propane. Both of these do not affect the ozone layer or add to global warming. There are concerns over toxicity and flammability of these refrigerants and so they should be used externally and according to appropriate guidelines. Absorption chillers which use a mix of ammonia/water and waste heat which would otherwise be wasted have a low contribution on global warming and ozone depletion when compared to other systems.

Heat Pumps

Heat pumps are vapour compression systems, but they are used for space heating rather than cooling.

It can be seen that what the vapour compression chiller is doing is extracting heat from a low temperature space and transferring it into an environment at a higher temperature. This is the basis of the heat pump (below figure) which uses the vapour compression cycle to absorb heat from outside air and convert it to higher grade heat for indoor space heating.

The theoretical efficiency with which the heat pump carries out this function is very high at approximately 300%. This means for every 1kWh of electricity put into the compressor 3kWh of heat is obtained by the building. In practice however the operating efficiency tends to be lower. This is for two main reasons. The first is that the highest efficiencies are obtained when the inside and outside temperatures are similar. This is not the case in winter when heat pumps are required for space heating. The second cause of the fall off in efficiency occurs on cold winter days when the evaporator may become iced up due to low temperatures. This restricts heat transfer across the evaporator. This can be avoided by using an electrical heater on the outside coil to defrost it or to reverse the refrigerant flow direction. Both of which reduce the overall efficiency of the device.

Even with this reduction in efficiency the efficiency with which the unit uses electricity to provide heating is higher than simple resistive heating. The operating efficiency of the device can be increased if a body of water is used as the heat source rather than the outside air. This is because the water will have a more stable and higher temperature than the surrounding air. Examples are canals, lakes, ground water or warm effluent.

Reverse cycle heat pumps are very useful pieces of equipment which can either heat or cool a space.

This feature is obtained by equipping the heat pump with a valve which can reverse the direction of refrigerant flow (below figure). The direction of the refrigerant flow determines if the coil inside the building is a cooling evaporator or heating condenser. Two expansion valves fitted with non-return valves are also required. Each expansion valve works in one direction only.

Reverse cycle heat pumps are particularly useful where spaces may have a requirement for both heating and cooling but at different times. One application is in shops where at the start of the day heating may be required. Later in the day as the shop fills with customers and heat is given out by display lighting, cooling may be needed to maintain comfort.

Absorption Chilling

There is growing interest in a method of cooling buildings which uses gas as a fuel instead of electricity. The technology is known as absorption cooling. The biggest differences between this and vapour compression cooling are that the compressor is replaced by a gas fired generator and the refrigerant is replaced by a refrigerant/ absorber mixture. A diagram of an absorption chiller is shown in below figure. The generator is filled with a mixture of refrigerant and absorber (solvent) which can be either water/lithium bromide (>35kW capacity) or ammonia/water. (>11kW). Note because water freezes the lithium bromide/water units can only cool down to 5oC, ammonia/water units on the other hand can cool down to -10oC. The way the system works can be illustrated using the ammonia/water pairing as an example. In this case water is the absorber and ammonia is the refrigerant. The water is called the absorber, giving the process its name, as it is so chemically attracted to ammonia vapour that it absorbs it out of the atmosphere.

A concentrated solution of ammonia in water is heated in the generator (figure 5.4) using a gas burner. The ammonia component vaporizes first, as it has a lower boiling point than water, and passes into the condenser. The water which is left behind passes back to the absorber. The ammonia vapour condenses back to liquid ammonia in the condenser giving out waste heat. This heat is removed from the system by air which is blown over the condenser by a fan. The ammonia now passes from the condenser into the evaporator via an expansion valve. In doing so its pressure drops and so it can evaporate once more. It does this by absorbing heat from the chilled water circuit. Chilling has therefore, been achieved.

The ammonia vapour now passes into the absorber where it is absorbed by the water from the generator to create a concentrated ammonia solution. Heat is given out when the two chemicals combine. This waste heat is also removed by the condenser cooling air flow. The ammonia solution is pumped back to the generator where the cycle continues once more.

The above device is known as a single effect absorption chiller. Double effect units are also available which use water and lithium bromide. This solution is pre heated on its way back to the generator by passing it through a heat exchanger. This improves the efficiency of the unit. Double effect units require a higher temperature heat source (>140oC) derived from a direct gas fired burner or pressurised hot water.

Absorption chillers are less efficient than vapour compression chillers with a COP of approximately 0.7-1.2. It follows that more gas energy will be required than an equivalent electric chiller (COP = 3.0). However the cost and pollution differentials will be reduced because electricity costs and pollutes approximately four times more per unit of energy than gas because of wastage in the power stations. Contract gas prices are lower still in summer when gas is needed for cooling as less is needed for space heating.

As well as direct gas firing some absorption chillers can be operated using waste heat. One form of surplus heat is that generated by combined heat and power units. In winter their heat output is used for space heating. In summer this heat is surplus to requirements and so can be used to drive the absorption chiller. This is known as trigeneration or combined cooling and power. When heat, which would normally be wasted, is used absorption chillers emit much less CO2 into the atmosphere than a vapour compression chiller for a given cooling effect. Research is currently underway which is investigating the linking of absorption chillers with solar panels as a source of generator heat. It is an advantage that the appearance of large amounts of cost and pollution free solar energy coincides with the need for cooling.

Other Benefits

The only electrical elements in an absorption chiller are the pumps used to move the ammonia/water solution from the absorber back to the generator. These pumps consume much less power and produce less noise and vibration than a compressor. This latter point is useful if the chiller is to be sited near to a noise sensitive area.

The pump along with the air cooled condenser fan and the gas burner fan are the only moving parts. The rest of the device consists of sealed metal chambers. This configuration means that maintenance costs are low.

External, air cooled, modular packaged units mean that cooling capacity can be easily expanded as the building is developed or as heat loads increase. Flexibility is further enhanced as units are also available that provide heating in winter and switch to cooling in summer.

Selection Criteria

From the above it can be seen that absorption chilling is particularly appropriate where:

· You have excess heat production from your CHP plant in summer or a production process which can be used to drive the absorption chiller

· The electrical supply to the site is not robust enough to supply the necessary electricity required for vapour compression chilling and an expensive upgrade would be necessary.

· You wish to optimize the use of clean gas as a fuel throughout the year, not just in winter.

· You have a source of low cost or free heat energy available such as solar energy or heat released from the combustion of landfill gas.