Ammonia AbsorptionRefrigeration Plant

D W Hudson, Gordon Brothers Industries Pty Ltd

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INTRODUCTIONRefrigeration plants using absorption principles have beenaround for many years with initial development taking placeover 100 years ago.

Although the majority of absorption cycles are based onwater/lithium bromide cycle, many applications exist whereammonia/water can be used, especially where lowertemperatures are desirable.

In both systems water is used as working fluid, but in quitedifferent ways: as a solvent for the ammonia-system, and as

refrigerant for the lithium bromide system. This explains thatthe lithium bromide absorption system is strictly limited toevaporation temperatures above 0ºC. On the other hand,water as a solvent in an ammonia absorption system has avapour pressure that requires rectification, whereas LiBr - ahygroscopic salt - is non-volatile and the desorbed watervapour is free from solvent without the need for purification.

The main industrial applications for refrigeration are in thetemperature range below 0ºC, the field for the binary systemammonia-water.

A typical ammonia absorption system is shown above.

ABSTRACT:

The application of ammonia absorption systems offers many advantages and this paper will review the basicoperating cycle, and investigate performance and economic benefits.

Keywords: Ammonia, absorption, refrigeration cycle.

THE OFFICIAL JOURNAL OF AIRAH - august 2002

Operation of the refrigeration cycle is conventional with highpressure liquid entering the liquid receiver from the condenserbefore passing to the evaporator where heat is absorbed fromthe process.

The remaining items in the system replace the conventionalcompressor to achieve “thermal” compression in three steps.

1. Absorption of the ammonia vapour in a weak ammonia-water solution at evaporation pressure.

2. Transport of the strong ammonia water solution from evaporator pressure to condenser pressure.

3. Removal of the ammonia from the ammonia-water solution(Desorption) at condenser pressure, together with the purification of the ammonia by heat energy.

High pressure ammonia leaving the fractionator column passes to the condenser for the cycle to continue.

It is important to note that the system must maintain athermal balance with total heat input balancing with thetotal heat rejected. This provides a simple check on theplant, as a variation would indicate a design error.

A description of each component in the plant follows:

a. A conventional evaporative condenser is used which in fact is slightly smaller than for a conventional plant. Ammonia vapour leaving the fractionator column is not highly superheated which allows the condenser to operate without the need for desuperheating.

b. The liquid receiver is conventional providing for variations in refrigerant volume flowing in the system.

c. A plate heat exchanger is used for glycol chilling duties asin a conventional plant.

d. The suction liquid heat exchanger E3 provides subcooling of the liquid and superheating of the suction gas. This heat exchanger is particularly important to improve the overall plant efficiency. The effect of a highly superheatedgas at the absorber is not critical.

e. The absorber is a critical plant item allowing the ammonia vapour to be absorbed into an ammonia water solution. Itis a combined heat and mass transfer problem where a considerable quantity of heat is liberated which must be rejected to the ambient air or a cooling water system. Careful design of the absorber is important and thin film techniques over a tube bundle are normal to handle the large differences in flow volumes between the ammonia vapour and the ammonia water solution.

Evaporating pressure is set by the absorber and if it fails to reject the required heat of solution under maximum operating conditions, then evaporator pressure and temperature will rise accordingly.

Total heat rejected by the absorber is much greater than for the evaporator duty and this depends to some extent

on the COP of the plant. Typically it can be over twice as much as the evaporating duty.

f. The absorbate receiver collects the strong ammonia water solution from the absorber for pumping.

g. A positive displacement pump or a high head centrifugal pump is used to lift the ammonia water solution from evaporator pressure to condenser pressure. The power required for this pump is only small as the volume flows are relatively low for the system. For industrial plants, thepower consumed is negligible and can be minimised by optimisation of the plant design.

h. The Aqua Heat Exchanger E1 reduces the temperature of the ammonia water mixture from the fractionator column and preheats the feed to the column. This heat exchangeris also important to increase the overall cycle efficiency.

In addition, for the absorber to work at peak efficiency, the weak ammonia-water solution must be as cool as possible.

Heat exchanger E2 removes heat from the steam condensate to add further heat to the column feed.

i. The fractionator column accepts the preheated feed from the Aqua-Heat Exchanger, where some desorption of the ammonia and water vapour occurs. It is divided into two sections each containing a fill material to aid the distillation process. The rectifying section is above the feed point and the stripping section is below the feed.

Liquid ammonia from the receiver is introduced as reflux into the top of the column to allow the ammonia vapour to be purified with a residual water content of 100 ppm orless. Reflux adds to the quantity of heat required for the column and must be kept to a minimum, consistent with maintaining an acceptable vapour quality at the outlet. All liquid ammonia introduced as reflux must be evaporated by heat from the reboiler. To offset the amount of heat for the reboiler a cold reflux from the absorbate pump can be used. This improves the heat ratioof the plant.

In the stripping section, ammonia is stripped from the ammonia-water solution, as it contacts ammonia and water vapour rising from the base of the column. The stripped solution is not allowed to mix with the weak ammonia-water solution in the base of the column, as a mixture would necessitate a higher column temperature. Instead the stripped solution passes to the reboiler (desorber) where heat is added to drive the process.

A considerable difference in temperature occurs over the column length with the highest temperature at the base, falling to condensing temperature at the top of the column. This fact allows heat to be introduced into the column at various levels, if desired.

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At the base of the column a weak ammonia-water solution is collected before passing to the Aqua-Heat Exchanger and absorber for re-use.

j. The reboiler accepts the stripped ammonia-water solution from the column and adds heat to drive the ammonia fromthe water.

It is possible to pump the ammonia-water solution through the reboiler, but this only adds unnecessary complications to the circuit.

During the heating process, some water vapour is driven off with the ammonia and this is why it is important for the fractionator column to remove as much moisture as possible using well designed reflux for the column.

The mechanical vapour compression system is shown belowto illustrate the differences in complexity of the two systems.

Mechanical Vapour Compression

CO-EFFICIENT OF PERFORMANCE

Co-efficient of performance (COP) is defined as the ratio ofrefrigeration effect, divided by the heat or power input. Foran Absorption system this is generally below unity, althoughit is theoretically possible to obtain figures well above unity.Losses in the system and thermodynamic irreversibilities willgenerally prevent a COP greater than one for refrigerationcycles.

The co-efficient of performance is also known as the HeatRatio in absorption systems and can be shown as follows:

Coefficient of Performance = Refrig Effect (Heat Ratio) Heat Input

Theoretical Carnot COP = (Tevap ) (Tgen - Tcond) (Absorption Plant) (Tcond - Tevap ) ( Tgen )

Theoretical Carnot COP = Tevap (Mechanical Plant) (Tcond-Tevap)

Tevap = Absolute evaporating temperature (k)

Tcond = Absolute condensing temperature (k)

Tgen = Absolute generator temperature (k)

It is important to note that the COP for an absorption plantis the same as for a mechanical plant but is modified by theCOP of the heat engine to drive the plant. If the COP of thepower generating station together with transmission losseswere added to the mechanical system then the overall COP ofboth systems become much closer.

Typical COPCalculation of COP that can be expected from an absorptionplant and a mechanical vapour compressor system is givenbelow.

Operating Conditions

Tevap = 253 K (-20ºC)

Tcond = 308 K (+35ºC)

Tabs = 308 K (+35ºC)

Tgen = 423 K (150ºC)

Carnot COPabs = 1.251 Practical COPabs = 0.6

Carnot COPmvc = 4.6 Practical COPmvc = 2.75

Tabs = Absolute absorber temperature (k)

Comparison of COP

Co-efficient performance is affected by evaporatingtemperature and from the above figure it is quite evident thatthe co-efficient of performance of the two systems are vastlydifferent. The COP for the absorption plant is much lessaffected by a drop in evaporating temperature and this is asignificant advantage in overall economy.

A number of other factors also affect COP in an absorptionrefrigeration plant and these are:

• Column temperature

• Ammonia purity (reflux ratio)

• Condensing temperature

• Cold reflux ratio

• Absorber temperature

• Heat exchanger effectiveness

Source TemperaturesThere is almost a direct relationship between the maximumtemperature of the fractionator column, the condensingtemperature and the evaporating temperature.

As the condensing temperature increases, the basetemperature of the column increases. Also as the evaporatingtemperature decreases, then the base temperature of thecolumn increases and this is shown below.

For low temperature applications, high grade waste heat isessential, but if this is not available, then economicoperation is quite possible using direct steam or gas heating.

It is also important to note the thermal gradient in thefractionator column means that heat can be added at lowertemperatures at strategic points in the column. This reducesthe quantity of heat required at maximum temperature, butmay enable other heating sources to be utilised.

OPERATING ECONOMICSEven though the thermal efficiency of the absorption plant isquite low, the direct operating costs must be considered inevaluating the plant.

Running Cost Comparisons (500 kW Plant)

The above figure compares the expected running cost perhour for a mechanical vapour compression system using powerat a cost of 8.5c/kWhr and also an ammonia absorptionrefrigeration system using steam generated from a boiler atcosts of $10, $15 and $20 per tonne of steam produced.

The upper curve is for $20 tonne of steam produced and atthis level the AAR plant cannot compare with a conventionalplant. However at lower costs of $15 and $10 / tonne theAAR plant does have lower running costs as the evaporatingtemperatures become lower.

CAPITAL COSTPreliminary estimates indicate that the ammonia absorptionplant would be 20% to 30% more expensive thanconventional equipment in sizes above 350 kW capacity. Thisonly compares equipment costs, and does not include servicessuch as transformers, electrical starters, electrical mains andbuildings, which are all required for a conventional plant, butare not required for the absorption system.

ADVANTAGESThere are a number of advantages for Ammonia AbsorptionRefrigeration Plants which need consideration:

1. The equipment produces no noise other than pumps, whichare small low power units.

Noise is a major problem with plant rooms, and with tighter specifications, conventional systems with screw compressors are difficult and expensive to attenuate.

2. There is no oil the system and the plant requires no oil management. In a new plant, oil will not contaminate heat exchange surfaces, which, if kept clean, will always deliver maximum performance.

Oil is always a problem in ammonia systems, with a continuous loss to the system occurring. Recovery is difficult and the oil is normally unsuitable for re-use in the system.

3. If waste heat is available, running costs are very low, as only condenser fans, aqua pumps and absorber fans need to be powered.

In a conventional system, the compressor will absorb up to 90% of the total power used. If waste heat is availableat the right temperature, then the power savings are very significant.

4. High turn down ratios are possible, giving efficient part load performance.

Generally speaking, all refrigeration plants are only sized for peak load conditions. For the majority of the time, plants run at part load, with low efficiencies. Screw compressors at high compression ratios have particularly poor part load characteristics, resulting in excessive powerconsumption.

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5. The number of moving parts is restricted to fans and pumps, which are easy to service and give long trouble-free life. Maintenance costs are low, as there are no expensive compressors to maintain.

6. Ammonia Absorption Plants can be fitted to existing systems to replace conventional compressors or supplement existing capacity.

7. The fractionating column and condenser can be located several hundred meters from the evaporator and absorber, with small interconnecting lines, which need not be insulated. This permits maximum flexibility in plant layout, without compromising overall efficiency. This is simply not practical with a conventional plant, due to system pressure drops.

8. For AAR plants, plant rooms are not required, as the absorber must be located outside and the fractionating column can be conveniently located at the heat source. Savings in building costs would be substantial.

9. As the demand for electric power is minimal, the need for large transformers and electrical mains, normally associated with a refrigeration plant, is eliminated. This cost is often overlooked in the cost of conventional refrigeration equipment and yet it is fundamental to the operation of the plant.

10.Liquid carryover is not a problem with Absorption Plants, as there are no moving parts to damage. In certain instances, liquid carryover from evaporators can be desirable, to prevent the build-up of water in the evaporator.

Liquid carryover in conventional plants is a major reason for compressor failures. Its causes are many, and even well maintained plants can suffer, due to sudden load changes or control malfunctions.

11.No upper limit for the size of the plant.

12.Waste heat can be conveniently converted to refrigeration,without the need for conversion to electrical energy.

DISADVANTAGESThere are some disadvantages with an ammonia absorptionplant and these are detailed below.

• Capital cost higher than mechanical plant

• More complex refrigeration system

• High grade heat required

• More space required

• Perception that it is outdated technology

CONCLUSIONThe Ammonia Absorption Refrigeration Plant offers a numberof advantages at temperatures below 0ºC and where wasteheat or cheap steam is available, significant running costssavings can be made.

With flexibility in operation, absence of compressor noise,very low maintenance and high reliability, industrial AmmoniaAbsorption Plants, should be considered as a viablealternative to mechanical vapour compression plants.

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