Despite its toxicity ammonia has a host of useful characteristics that make it a suitable alternative refrigerant. We look at some of its advantages as well as the approaches for minimising the hazards.
Since the phasing out of cfcs and hcfcs, the air conditioning industry has endeavoured to find alternative refrigerants. HFCs are convenient and safe options, but they are still manufactured synthetic products and are powerful greenhouse gases. Thus, hfcs have been included in the Kyoto Protocol as controlled greenhouses gases, and the UK Climate Change Programme states that hfcs should only be used where other safe, technically feasible cost-effective and more environmentally acceptable alternatives do not exist.

HFCs are not a long-term option, therefore other refrigerant options must be looked into. Ammonia is one of the most commonly quoted options, and in spite of certain disadvantages, it has useful characteristics.

Properties of ammonia
Ammonia has properties that are quite different to the conventional halocarbon (for example cfc, hcfc and hfc) refrigerants. Firstly, ammonia is flammable in air at relatively high concentrations, between 15% and 28%. In comparison most hcfcs and hfcs (with a few exceptions) are not flammable except when mixed with air at high pressure. Although explosions have occurred after large accidental releases of ammonia from industrial plant such mishaps are exceedingly rare and appear to be almost invariably due to bad practice. For example, the HSE1 is only aware of three ammonia /air explosions between 1974 and 1994 in the UK. The HSE considers that systems installed and operated according to current safety codes and standards should be safe. The most important are BS EN 378: 2000 (Refrigerating systems and heat pumps – safety and environmental requirements)2, and the Institute of Refrigeration's safety code for compression refrigerating systems utilising ammonia3 which amplifies and provides additional information to that in BS EN 378.

All mainstream hcfc and hfc refrigerants have very low toxicities, except at very high concentrations. However, ammonia is toxic at much lower concentrations. The UK Occupational Exposure Standard is 25 ppm for an eight hour time weighted average, which is the maximum that workers should be exposed to during an eight hour working day. However, this limit ignores some important mitigating factors. Most people can readily smell ammonia at less than 25 ppm and by 50 ppm ammonia has an obvious pungent smell making its presence self alarming. Between 50 ppm and about 200 ppm ammonia vapour is unpleasant and causes lung and eye irritation but no known permanent or chronic health effects.

Although in the past ammonia was considered to be a high pressure refrigerant it has operating pressures very similar to other mainstream hcfc and hfc alternatives being used or considered for air conditioning type applications. R410a which is now being used for some packaged air conditioning systems has a much higher condensing pressure.

For many reasons, including energy efficiency, it is desirable to operate at low compressor pressure ratios. The pressure ratio for ammonia systems is very similar to comparable hcfcs and hfcs for the same evaporating and condensing conditions.

The critical temperature and pressure of a refrigerant define the point above which the refrigerant behaves like a permanent gas which cannot be liquefied. As the critical temperature is approached, the latent heat of vaporisation and therefore the efficiency of the refrigerant cycle is reduced. Therefore the condensing temperature should not approach the critical temperature of the refrigerant. Ammonia has a particularly high critical temperature which contributes to the typical good energy efficiency of ammonia refrigeration systems. A high critical temperature would also make ammonia suitable for high temperature heat pumps.

The specific heat ratio of a refrigerant vapour (the ratio of the specific heats at constant pressure and constant volume), determines the index of compression and hence the temperature rise during compression. Ammonia has a relatively high index of compression which causes high compressor discharge temperatures which on its own is normally considered to be a disadvantage. However, in practice this can be overcome by water cooled cylinder heads in reciprocating compressors or oil injection and oil cooling in screw compressors. In some situations high discharge temperature can be an advantage such as when heat recovery is required. With desuperheating high grade heat recovery up to about 80°C becomes possible. For air conditioning system operating temperatures the high discharge temperature of ammonia is not really a problem.

The latent heat of vaporisation determines the mass flow of refrigerant required for a particular refrigerating duty. High latent heat is a desirable refrigerant property, although it can be offset by other factors. Ammonia has an extraordinarily high latent heat, six times that of R22, R407c and R134a. This results in reduced refrigerant charge and smaller pipes and vessels which is a significant advantage for large systems, although below 10 kW pipe sizes become inconveniently small.

As well as very high latent heat ammonia has low viscosity and high liquid thermal conductivity and these combined properties result in ammonia having very high heat transfer coefficients. The benefits of this can be exploited either by making heat exchangers smaller and therefore cheaper, or by using the same size heat exchangers and benefiting from a higher evaporating temperature and a lower condensing temperature and the energy efficiency advantages that this entails. Ammonia vapour has a relatively low density which unfortunately more or less completely offsets the advantage of having a high latent heat on the size of compressor. The refrigeration capacity per cubic metre of ammonia is similar to R22 and R407c and therefore requires similar sized compressors.

Very high latent heat gives ammonia another advantage. When ammonia is reduced to atmospheric pressure it tends to subcool because there is usually insufficient heat available in the surroundings to cause it to evaporate all at once. This means that large liquid releases tend to form a pool of liquid ammonia on the floor which then slowly evaporates. (It should be noted that a significant danger exists if fire fighters or other personnel drench spilt liquid ammonia with water. The available heat in the water can cause violent boiling and formation of dense clouds of ammonia vapour with liquid droplets). The same sub-cooling effect also occurs when there is a containment failure, such as a ruptured pipe or vessel, and a substantial proportion of the ammonia charge will initially stay inside the machine. Practical experience5 suggests that only 10% of the liquid volume remaining in the system or spilt on the floor will evaporate provided that heat is not added to it. These are very important hazard mitigating factors. Ammonia may at first sight appear to be a very dangerous substance but many of its thermo-physical properties work to make it far safer than it first appears.

A disadvantage of having a very high latent heat is that there is a risk of refrigerant distribution problems in direct expansion (dx) evaporators with thermostatic expansion valves, especially at low load. Over feeding the evaporator can cause excessive liquid carry over because not enough heat can be supplied to vaporise all of the ammonia, with consequential risk of compressor damage. Also, ammonia is immiscible with standard oils and in dx evaporators this can result in poor oil return to the compressor and oil clogging of heat exchanger surfaces. Several experienced ammonia system manufacturers warn about the risk of using direct expansion evaporators with ammonia6,7,8. The industry appears divided on this issue because several other manufacturers offer a wide range of dx ammonia chillers using electronic microprocessor controlled expansion valves and synthetic miscible oils.

Ammonia is not soluble and not miscible with traditional mineral oils and this is the reason why older industrial ammonia plants required labour intensive manual oil draining and replenishment of the compressor oil. Newer hydro-cracked mineral oils and polyalphaolefin synthetic oil, while not miscible with ammonia, have much reduced viscosity especially at low temperatures and this has allowed automatic oil return systems to be developed. Newer synthetic oils based on polyol alkylene glycols are being developed that are completely miscible with ammonia but long-term experience with them is limited at present. These oils have low viscosities which has to be taken into account in compressor design and are hygroscopic which requires changes in maintenance practices.

Ammonia is compatible with steel and aluminium but not copper or alloys containing copper such as brass. In comparison cfcs, hcfcs and hfcs are compatible with all of these metals. Ammonia systems are normally constructed from steel pipe and are usually welded. This tends to raise the cost of ammonia systems and reduce the choice of components. On the other hand ammonia systems using welded steel piping tend to be much more robust.

Minimising the hazards of ammonia
In addition to meeting the requirements of industry standards and codes it is recommended that ammonia systems installed for air conditioned buildings are either installed in a special machinery room within the building or inside a special enclosure which may be on the building roof. The machinery room or container should comply with the requirements for a special machinery room set out in BS EN 378. The advantages of such a machinery room or container is that any spilled or leaked liquid ammonia can be contained and that external gas discharge rates can be controlled.

Although explosions have occurred after large accidental releases of ammonia such mishaps are exceedingly rare.

It is a common mistake to assume that ammonia vapour will discharge as a buoyant plume because ammonia vapour is lighter than air. In reality wind effects can cause ammonia vapour to discharge as a neutrally buoyant plume with a risk of entering air inlets on the roof or being entrained along the downwind face of the building. Further information on this has been published by BRE9. Discharge should be through a fan-assisted stack with high velocity efflux to give the plume as much vertical momentum as possible. Relief valve lines should similarly discharge upwards and at as high a velocity as possible10. High velocity discharge will throw the plume as far as possible from the building and ensure maximum dilution through entrainment of ambient air. Note that to maximise the velocity of the discharge the diameter of the relief valve line should be as small as possible consistent with the back pressure limits of the relief system. Alternative practical approaches used in the UK for relief lines have included discharge into the plant ventilation extract downstream of the fan.

The quantity of ammonia refrigerant charge for a given installed cooling capacity should be kept as low as practically possible. This effectively rules out the use of pumped liquid recirculation systems with flooded evaporators, which have been popular for industrial ammonia systems. Heat exchangers should be designed for minimum charge and for this reason plate heat exchangers have grown in popularity for both evaporators and condensers. In buildings with very high cooling loads it is desirable to use multiple chillers as this reduces the maximum ammonia charge contained in any one chiller, and also improves part load matching and energy efficiency.

Traditional practice suggests that highly sensitive ammonia detectors are not required in plant rooms because the smell alerts operators to the tiniest of leaks. However, this is based on traditional industrial ammonia installations where technically competent staff are usually in attendance most of the time. Chillers used for commercial air conditioning may well operate unattended for relatively long periods and for these applications sensitive ammonia detectors are required to alert building operators to possible ammonia leaks and to prevent inadvertent access to an ammonia gas filled plant room. For these installations care is needed in the specification of the ammonia detectors to chose sensors with good reliability and low drift. BS EN 378 requires a first alarm level of 500 ppm to activate an alarm and emergency ventilation fans, followed by complete stoppage of the refrigerating system at 3% (30 000 ppm) to prevent the risk of explosion. It might be appropriate to use a lower alarm threshold, although this would increase the risk of nuisance false alarms.

Various types of ammonia detector technology are available, including semiconductor and electrochemical sensors and infrared analysers. Semiconductor and electrochemical sensors are sensitive to other gases while infrared systems are more expensive and require regular recalibration. Whichever type of detection technology is used it must not be considered to be a fit and forget item. All detectors require period recalibration and electrochemical sensors have a short lifetime.

Atmospheric dispersion of leaked or spilt ammonia is normally the best form of disposal. Such emissions are insignificant compared to natural or agricultural sources. However, in very sensitive locations it is possible to almost completely avoid ammonia discharges by absorbing ammonia in water. Ammonia can be absorbed into water in a tank at the rate of approximately 100 kg to 120 kg of ammonia per cubic metre of water. Another approach is to either use a water spray or a water and acid absorption column11.

Ammonia chiller technology
Industrial ammonia refrigeration systems commonly use pumped recirculation systems with flooded evaporators. However, the high refrigerant charge of these systems makes them unsuitable for building air conditioning applications for which higher safety standards are necessary. Chiller manufacturers appear to be offering two basic types of ammonia chiller for building air conditioning, low pressure receiver (lpr) systems and dry expansion (dx) evaporator systems with thermostatic expansion valves. There appears to be differing views within the industry about which system type is best, with one quite widely held view that dx systems are difficult to control and are at risk of liquid slugging the compressor, and have poor oil circulation. The development of microprocessor expansion valve control coupled with synthetic oils that are miscible with ammonia may have overcome these old problems but further documented experience is needed to give manufacturers confidence that the good safety and reliability reputation of ammonia will not be compromised. No doubt as experience with dx systems increases there will be significant developments in this area in the near future.

The low pressure receiver (lpr) offers the ability to run evaporators with fully wetted surfaces and therefore high heat transfer effectiveness, but with lower charge than normal flooded evaporator designs and without the need for a liquid circulation pump. LPRs are used by several ammonia chiller manufacturers and have been used in recent ammonia chiller installations for air conditioned buildings, including a merchant bank in the City of London. The operation of the lpr has been covered in previous papers12 but is summarised in figure 1. The lpr is essentially a liquid receiver located between the evaporator and the compressor suction but also contains a heat exchanger. This heat exchanger transfers heat between the high pressure liquid line and the low pressure suction line. This evaporates low temperature liquid refrigerant in the lpr so that dry vapour only reaches the compressor suction, and also provides further sub-cooling of the liquid line. However, because the heat exchange is internal within the refrigerant cycle there is no thermo-dynamic benefit from the increased subcooling. LPRs have recently been incorporated into thermosyphon chillers13, resulting in extremely high efficiency systems. However, thermosyphon systems require quite high refrigerant charges and this is not desirable with ammonia for air conditioning.

Advantages of lpr systems
LPR systems have a number of advantages. They act as a liquid receiver, storing excess refrigerant charge and preventing liquid flooding back to the compressor. They enable the evaporator to be as efficient as possible by operating in a flooded manner. They also allow evaporator operation with low temperature differences because there is no need for a superheat signal at the evaporator exit, which improves efficiency and reduces the size, weight and cost of the evaporator compared with dx systems. As well as that they can be used with simpler more reliable and stable expansion valve systems such as high pressure float switches, instead of potentially unstable and unreliable ammonia thermostatic valves. LPR systems also allow the collection of excess lubricating oil which can be returned directly to the compressor suction without passing through the evaporator which avoids excessive fouling of the evaporator surfaces.

A major disadvantage of lpr systems however is that they add an extra heat exchanger to the chiller although the extra cost of this is offset by reduced energy consumption over the life-time of the system.

Future developments
A number of developments are focusing on very low charge ammonia systems. For example, work at the University of Illinois at Urbana-Champaign is developing reduced charge air-cooled ammonia chillers14. By using aluminium microchannel tubing to construct the air-cooled condenser, the researchers have managed to produce and test an ammonia chiller with a specific charge of 0·018 kg/kW as against current industry good practice of around 0·1 kg/kW. A 100 kW chiller would therefore have an ammonia charge of only 1·8 kg. Development of commercial machines could revolutionise the ammonia refrigeration industry and make its widespread use for air conditioning more likely.

Round-up
Although the number of ammonia chillers installed for building air conditioning is currently quite small it has been shown that ammonia has excellent refrigeration properties. At first sight ammonia appears to be a difficult and dangerous refrigerant to use but in fact its properties mitigate its toxicity and flammability.

A major barrier to the uptake of the use of ammonia refrigeration for buildings is an apparent lack of confidence and experience by building services designers, consultants and contractors. This situation should improve as the number of ammonia chillers used for air conditioning increases. It is important that information from these installations is made available to the industry by disseminating and publicising it as widely as possible.

Another barrier to the wider uptake of ammonia refrigeration in buildings is the higher initial cost compared to comparable hfc systems. However, when life cycle cost are taken into account ammonia may be the cheapest option on account of ammonia being a particularly good refrigerant and ammonia chillers having lower energy consumption. This needs designers and specifiers to be well acquainted with the benefits of ammonia refrigeration and the life cycle cost analysis methods. Building owners also need to take a longer term approach and take account of environmental impact and sustainability in their purchasing decisions.

Future developments are likely to result in ammonia chillers with lower refrigerant charge than possible at present, while maintaining the high reliability and stability of current ammonia chiller designs. When this happens there is no reason why ammonia should not become a refrigerant of choice for the larger chilled water based air conditioning systems.

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