Ammonia based refrigeration plants are exceptionally well suited for industrial sites as the volatility of ammonia is so readily taken advantage of to suit both heating and cooling requirements. Unfortunately, there are also some unfavourable characteristics which must be addressed in order to ensure the system remains reliable and effective. Ingress of foreign substances to the system can have a variety of effects on components, but all share the commonality of a decrease in performance.
By a wide margin, the most common contaminants in industrial refrigeration systems are air and water; each requiring actions be implemented to mitigate their influence on the system. Whether by inadvertent maintenance actions or by failing equipment, a robust system should have contingencies in place to combat these effects.
Hygroscopic substances are those which actively absorb moisture from their surroundings. The most common occurrence in our everyday lives is seen in the silica gel packets which come packaged in with everything from new shoes to food wraps. The goal of these sachets is to dehydrate the surrounding environment by absorbing moisture. As with silica, Ammonia also demonstrates this hygroscopic trait, but it’s within the carefully controlled environment of a refrigeration system that excess water can create problems.
Ammonia plants will readily absorb any moisture they are exposed to through leaky seals, improper maintenance, or failed components. A small leak may take some time to become evident; constant vigilance is the best approach to ensure a long, efficient system life.
Many of the components within these systems are constructed from iron, which will readily corrode when exposed to moisture. Normally, the redox reactions required to take place, where we see the pitting effects of corrosion, cannot occur as ammonia cannot act as ion transport medium. The formation of surface rusting will take many years to corrode through a system walls but the more immediate effects of increased surface roughness can have significant impacts upon the energy losses within the system.
Figure 1 – The forbidden attraction of ammonia and water can wreak havoc on refrigeration systems
Water presents another major potential issue as the freeze point is completely incompatible with the conditions experienced within the refrigeration process. Vapour-compression systems, by necessity, require compression and expansion to take place to create a cooling effect. When water is exposed these conditions it will too readily freeze and create blockages in instruments which are difficult to remove. This is especially a problem for control valves which operate using small orifices that can become blocked easily. Once frozen, the component must be warmed up or removed to clear, which increases downtime and cost.
Most ammonia systems operate at a pressure greater than atmospheric pressure. This characteristic of the plant has the benefit that any system leaks will cause an egress of ammonia, not ingress of impurities. For the most part, this keeps air out of systems where proper maintenance procedures are followed.
On low temperature systems, the operating pressures are substantially reduced in order to achieve the desired condition. Necessarily this means that the compressor suction pressure is reduced to such an extent that a condition of vacuum is realised (pressure below atmospheric pressure). It is the existence of this vacuum which very effectively draws air into the system though any slight leaks, whether by improper seals or failed equipment.
The result of air (foul gas) in the system is that it begins to occupy space which would be otherwise available to the ammonia. Air is also wet/humid, and therefore the ingress of air will also bring in moisture.
The largest influence of a cooling system is it’s ability to efficiently transfer heat from one area to another, and the best way to achieve this energy transfer is by convection. In order to maximise the energy absorption potential of ammonia, we take advantage of the latent heat of evaporation which has a much greater energy absorbing requirement when compared with sensible heating. Therefore, if we increase the available surface area for the ammonia liquid to spread across, more of the energy will be absorbed in evaporation and create a more efficient system.
Impurities in the system, such as air, can potentially take up space in all pieces of equipment where heat transfer is occurring. By taking up space which would otherwise be utilized by ammonia, heat is instead transferred into the (already gaseous) air. This heating of air is a far less efficient use of heat transfer and thus the efficacy and efficiency of the system is reduced.
These gasses have a tendency to accumulate at certain zones within system, therefore appropriately placed purging lines can be installed to syphon off impurities.
In order to remove the moisture and foul gas from a system, the best approach is one which constantly acts in order to remove trace amounts before they can impact the plant operation. Air-purgers specifically designed for use with ammonia systems are readily available and should be considered for all ammonia installations to ensure the longevity of the system. Foul gas collectors come in various sizes and configurations dependent on the size of the system. A basic representation of the foul gas removal process is shown in Figure 2.
Figure 2 – Basic Air Purging unit
The operation, simplified, of the air purger is as follows:
More sophisticated units also have the ability to remove water from the system by utilising an additional evaporator compartment which removes moisture from the vapour stream before separating other foul gasses. These units are ideal for large installations where water can collect and hide easily amongst field components, making it more difficult to extract by other means.
Figure 3 – A small air purger in the field
The investment of an air purger of an industrial system is well worth the cost of installation, with a typical payback period well under one year of operation. Benefits are realised on many plant attributes such as a consistent high plant efficiency, reduced energy consumption and reduced maintenance costs associated with manual intervention to remove impurities.