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Engineers have the choice of older designs using flooded evaporators (above) and the well-established direct-expansion evaporator. |
Ammonia is a refrigerant for today, not yesterday. Geoff Lovell runs us through getting the best from this long-established refrigerant.
Ammonia is now firmly re-born into air conditioning. All fashions, if you wait long enough, always return — in this case not for vogue but in response to pressure on environmentally friendly refrigerant solutions to the current alternatives to CFCs, HCFCs and, yes, HFCs!
Their global re-acceptance faces just one difficulty. Most building-services designers and engineers who once commonly worked with ammonia, until apparently safer CFCs took over in the 1950s, have retired, leaving a modern generation bewildered by this new(!?) environmentally benign but potentially flammable and toxic alternative, even though ammonia has been used in refrigeration for over 120 years.
Fortunately, manufacturers not only retained their knowledge but have improved upon it, using modern advanced technologies to solve previous difficulties with refrigerant-flow control and to deal with safety issues.
As always, it is the manufacturers which influence the design of equipment, based upon factors such as their production bias, cost or just plain design preference.
This is already becoming evident in the ammonia field, with key manufacturers polarising upon their design choice of evaporator and refrigerant feed, and endeavouring to sway the services designers to their preference. This is inevitably causing both confusion and fear of specifying the unknown and not helping the good case for changing to environmentally friendly ammonia systems.
Today’s services designers are already being called on to choose between older designs using flooded evaporators and the well established direct-expansion evaporator, which has become the standard for all today’s modern HCFC and HFC refrigeration water chillers.
The question therefore is which is best? The answer is that there is no best, only advantages and disadvantages that must be considered, primarily these are based on:
• cost;
• reliability;
• size;
• efficiency.
Past historic
Early ammonia designers faced difficulties with refrigerant-flow control and oil return.
The first issue, refrigerant-flow control, was due to the very high latent heat offered by ammonia (six times that of HCFC 22), which has the advantage of reducing the refrigerant charge and pipe sizes. However, when used with direct-expansion systems a risk was created of incompressible liquid entering the compressor, with disastrous results. A solution was to use flooded systems, which could reduce risk and improve heat exchange efficiency but which created a new problem of returning the lubricating oil inevitably lost during compression back to the compressor, as it would remain within in the evaporator.
Today’s advances
Fortunately, with the general growth of micro-processor technology, greatly improved refrigerant-flow controls came along, specifically the ‘electronic expansion valve’ for use with direct-expansion systems. These valves replaced thermal expansion valves and offered the advantage of allowing very close control of the refrigerant feed to the evaporator, resulting in several benefits.
• A reduction of the superheat needed to prevent liquid to return to the compressor suction, reducing from 5 K needed for thermal expansion types to 1 K or less with electronic expansion valves.
• A reduction of the evaporator area being used inefficiently for superheating (often 15 to 20% to achieve 5 K), which increases heat transfer to nearer to flooded system values.
• Re-application of direct-expansion systems with good refrigerant-gas-flow velocities within the evaporator to force oil back to the compressor, without need for complex separate oil-return systems.
This comparatively simple device has removed the previous barriers for selecting direct expansion evaporators for use with ammonia systems, providing the advantages over flooded evaporator systems.
• Smaller equipment.
• Reduced cost.
• Easily understood refrigeration technology for maintenance engineers, similar to that used in HCFC and HFC equipment.
• High reliability.
• Greatly reduced refrigerant charge.
• No oil loss problems.
• No complicated refrigerant float valve controls.
Other evaporator options
As if the choice between dry-expansion and flooded evaporators is not enough for building-services designers to think about, enter the LPR system (low-pressure receiver). This system is an enhancement of the flooded design and basically using a receiver placed between the evaporator and compressor to evaporate otherwise wet refrigerant leaving the evaporator to dryness before it reaches the compressor. Superheating of the refrigerant is achieved not in the evaporator but in the receiver, making it possible for the evaporator to operate in a flooded manner.
The action of passing the hot refrigerant liquid through the receiver for superheating further creates sub-cooling of liquid before it enters the evaporator and improves efficiency.
Sub-cooling in conventional systems, where the cooling is provided from an outside source, greatly enhances the efficiency of the refrigerant cycle. However, as the LPR system exchanges the heat internally within the refrigeration circuit, there is no direct improvement in system efficiency, albeit there are benefits.
The effect of sub-cooling means that wetted refrigerant leaving the evaporator is returned or is re-circulated to the receiver (overfeeding), reducing the proportion of flash gas entering the evaporator and ensuring fully wetted heat exchange within the evaporator. This can provide increased efficiency over a direct expansion system but would normally only account for a COP improvement of about 2.5%.
The LPR system offers an advantage over conventional flooded systems in that the refrigerant charge is minimised. However, for overfeeding to occur and provide the benefits described above, there must be an excess of refrigerant flowing through the evaporator to that flowing through the compressor; with the high latent heat of ammonia, this does not generally occur.
The LPR system uses a refrigerant high-pressure float control device located on the condenser outlet. This system of control is uncommon in traditional air-conditioning water chillers.
The factors for consideration of the LPR system can be summarised as follows.
• Slightly higher efficiency than direct-expansion systems.
• Utilisation of a more complex refrigeration systems, which may cause serviceability issues for today’s air-conditioning engineers and reduce owners’ ability to obtain competitive service contracts.
Increased difficulties in ensuring oil return under all conditions similar to flooded evaporators, as oil cannot be returned to the compressor through the suction line but by external sources.
Building-services designers whose expertise has to cover a broad range of disciplines need not become refrigeration-system experts, but should set the performance parameters they require for their application (capacity, turndown, efficiency etc.) and allow manufacturers to put forward their products and benefits and, above all else, demonstrate that they have both the engineering history and operating experience to allow consideration.
Ammonia is not just coming to air conditioning — it has arrived. With technology advances, it has become easy to apply and more cost effective. It is important for both designers and end users that the systems are both reliable and that these technologies can be absorbed by the existing air-conditioning service and maintenance providers. The old adage ‘let’s keep it simple’ rather than chase the perfect solution applies well here.
Klima-Therm, which represents RVR (Ramon Viscaino Refrigeracion) has recently installed six ammonia direct-expansion water chillers, each rated at 1000 kW. It is probably the largest air-conditioning application in London, if not the UK.
RVR was established in 1932 and manufactures only ammonia-based products as part of its commitment to an environmental policy to eradicate the use of ozone-depleting and global-warming refrigerants. The company has installed hundreds of ammonia-based machines throughout Europe and, indeed, the world, and has over 100 designers amongst its 400-plus team, dedicated to design and development — testimony to its qualification to satisfy owners and designers of its competence in this specialist field.
Geoff Lovell is a consultant with Klima-Therm (Distribution) Ltd, Unit 10, Brooklands Close, Sunbury-on-Thames, Middx TW16 7DX.
