Ammonia Absorption

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Ammonia Absorption Refrigeration Plant D W Hudson, Gordon Brothers Industries Pty Ltd ABSTRACT: The application of ammonia absorption systems offers many advantages and this paper will review the basic operating cycle, and investigate performance and economic benefits. Keywords: Ammonia, absorption, refrigeration cycle. INTRODUCTION Refrigeration plants using absorption principles have been around for many years with initial development taking place over 100 years ago. Although the majority of absorption cycles are based on water/lithium bromide cycle, many applications exist where ammonia/water can be used, especially where lower temperatures are desirable. In both systems water is used as working fluid, but in quite different ways: as a solvent for the ammonia-system, and as

refrigerant for the lithium bromide system. This explains that the lithium bromide absorption system is strictly limited to evaporation temperatures above 0ºC. On the other hand, water as a solvent in an ammonia absorption system has a vapour pressure that requires rectification, whereas LiBr - a hygroscopic salt - is non-volatile and the desorbed water vapour is free from solvent without the need for purification. The main industrial applications for refrigeration are in the temperature range below 0ºC, the field for the binary system ammonia-water.

A typical ammonia absorption system is shown above.

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THE OFFICIAL JOURNAL OF AIRAH - august 2002

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Operation of the refrigeration cycle is conventional with high pressure liquid entering the liquid receiver from the condenser before passing to the evaporator where heat is absorbed from the process. The remaining items in the system replace the conventional compressor to achieve “thermal” compression in three steps. 1. Absorption of the ammonia vapour in a weak ammoniawater 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 a thermal balance with total heat input balancing with the total heat rejected. This provides a simple check on the plant, 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 as in 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 superheated gas 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. It is 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, the power 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 exchanger is 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 or less. 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 ratio of 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.

THE OFFICIAL JOURNAL OF AIRAH - august 2002

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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 from the 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 below to illustrate the differences in complexity of the two systems. Mechanical Vapour Compression

Tgen = Absolute generator temperature (k) It is important to note that the COP for an absorption plant is the same as for a mechanical plant but is modified by the COP of the heat engine to drive the plant. If the COP of the power generating station together with transmission losses were added to the mechanical system then the overall COP of both systems become much closer. Typical COP Calculation of COP that can be expected from an absorption plant and a mechanical vapour compressor system is given below. 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 OF PERFORMANCE Co-efficient of performance (COP) is defined as the ratio of refrigeration effect, divided by the heat or power input. For an Absorption system this is generally below unity, although it is theoretically possible to obtain figures well above unity. Losses in the system and thermodynamic irreversibilities will generally prevent a COP greater than one for refrigeration cycles. The co-efficient of performance is also known as the Heat Ratio in absorption systems and can be shown as follows: Coefficient of Performance = (Heat Ratio)

Refrig Effect Heat Input

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

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

Theoretical Carnot COP = (Mechanical Plant)

• Column temperature

Tevap (Tcond-Tevap)

Tevap = Absolute evaporating temperature (k) Tcond = Absolute condensing temperature (k)

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Co-efficient performance is affected by evaporating temperature and from the above figure it is quite evident that the co-efficient of performance of the two systems are vastly different. The COP for the absorption plant is much less affected by a drop in evaporating temperature and this is a significant advantage in overall economy.

THE OFFICIAL JOURNAL OF AIRAH - august 2002

• Ammonia purity (reflux ratio) • Condensing temperature • Cold reflux ratio

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• Absorber temperature • Heat exchanger effectiveness Source Temperatures There is almost a direct relationship between the maximum temperature of the fractionator column, the condensing temperature and the evaporating temperature. As the condensing temperature increases, the base temperature of the column increases. Also as the evaporating temperature decreases, then the base temperature of the column increases and this is shown below.

The above figure compares the expected running cost per hour for a mechanical vapour compression system using power at a cost of 8.5c/kWhr and also an ammonia absorption refrigeration system using steam generated from a boiler at costs of $10, $15 and $20 per tonne of steam produced. The upper curve is for $20 tonne of steam produced and at this level the AAR plant cannot compare with a conventional plant. However at lower costs of $15 and $10 / tonne the AAR plant does have lower running costs as the evaporating temperatures become lower. CAPITAL COST Preliminary estimates indicate that the ammonia absorption plant would be 20% to 30% more expensive than conventional equipment in sizes above 350 kW capacity. This only compares equipment costs, and does not include services such as transformers, electrical starters, electrical mains and buildings, which are all required for a conventional plant, but are not required for the absorption system. ADVANTAGES There are a number of advantages for Ammonia Absorption Refrigeration Plants which need consideration: 1. The equipment produces no noise other than pumps, which are small low power units.

For low temperature applications, high grade waste heat is essential, but if this is not available, then economic operation is quite possible using direct steam or gas heating. It is also important to note the thermal gradient in the fractionator column means that heat can be added at lower temperatures at strategic points in the column. This reduces the quantity of heat required at maximum temperature, but may enable other heating sources to be utilised. OPERATING ECONOMICS Even though the thermal efficiency of the absorption plant is quite low, the direct operating costs must be considered in evaluating the plant. Running Cost Comparisons (500 kW Plant)

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 available at 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 power consumption.

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FORUM 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.

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

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

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

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

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.

DISADVANTAGES There are some disadvantages with an ammonia absorption plant and these are detailed below. • Capital cost higher than mechanical plant • More complex refrigeration system • High grade heat required

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.

• More space required • Perception that it is outdated technology

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.

CONCLUSION The Ammonia Absorption Refrigeration Plant offers a number of advantages at temperatures below 0ºC and where waste heat or cheap steam is available, significant running costs savings can be made. With flexibility in operation, absence of compressor noise, very low maintenance and high reliability, industrial Ammonia Absorption Plants, should be considered as a viable alternative to mechanical vapour compression plants.

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.

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THE OFFICIAL JOURNAL OF AIRAH - august 2002