The ever increasing energy cost has indeed been pushing the industry to look for energy saving methods. Refrigeration system owners too are keen to use appropriate methods to save energy _ since it constitutes a major running cost. As a commitment, the refrigeration system users too are committed to a smaller carbon foot print, which has assumed a greater significance. The energy cost of refrigeration system of a typical super market, for example, constitutes about 60 to 70% of total running cost. And therefore in such a case, the energy cost saving can add to the bottom line of the owner.
In order to achieve or realise an effective cost reduction, a concerted, properly planned effort is required on the part of an organization. It is generally noted that a new project undergoes an intensive scrutiny as far as the capital cost is concerned _ but a little attention is paid to the future cost of running the plant.
In a scenario like this, at times the buyer ends up with old technology products that are not very energy efficient. Besides, the auxiliary system equipment like cooling tower, condenser water pumps, piping installation need careful attention, and so does the post commissioning maintenance to realize the benefits of energy efficiency. All in all, a careful equipment selection, its proper installation and maintenance are the hallmarks of a good system.
There is a great scope for improving the energy efficiency of refrigeration systems. In addition to the reduction of energy costs, it has the added benefit of increasing the reliability and reducing service and downtime costs throughout the plant`s life. The energy saving steps will vary from application to application, however, an attempt is made here to generalize steps, which would lead to energy cost reduction in refrigeration systems.
Key points are summarised for better understanding. These are as under :
Evaporator performance is governed by
the temperature difference between the medium being cooled & suction saturation temperature (refrigerant temperature) in evaporator. The higher the temp difference, greater will be the heat transfer
size & design of evaporator.
Condenser capacity is affected by
Cooling medium
Size and design of condenser
Compressor capacity is affected due to
Compressor size or simply displacement in metre cube per hour.
Compression ratio
Temperature of vapour entering the compressor
Refrigerant characteristics.
Thumb rules
An increase in evaporator temperature by 1 deg C results in an increase of compressor COP by about 2 to 4%
A decrease in condensing temperature by 1 deg C also has a positive effect on COP which improves by 2 to 4%
Evaporator efficiency issues
In order to maintain higher evaporator efficiency, evaporator temperature should be as high as possible. Use of a larger evaporator can achieve this but in addition
keep away dirt & slime on evaporator coil surface.
Clean regularly (in shell & tube evaporators), scale, corrosion & deposits of dust/foreign particles.
Carry out regular maintenance of equipment (water circulating pumps/ blowers) which circulate cooling medium.
Always keep oil out of evaporator, it lowers efficiency & COP of system
Abnormal superheat.
Compressor efficiency issues
Operation on an in-built capacity control should be avoided or minimised wherever possible. Methods to achieve this are:
Avoid single large capacity compressor
Select compressors in such a combination that it avoids a need to operate one or more compressors on capacity control
On multiple compressor applications, attempt to minimise the use of compressors on capacity control. Example, do not design a system whereby two compressors operate on 50% capacity simultaneously. Rather operate one compressor on 100% load.
Condenser efficiency issues
Three different types of condensers are used in refrigeration system; each of these have been associated with energy consumption. The energy should be accounted.
Air-cooled: Fan consumes the energy
Water-cooled: Condenser water pump, cooling tower
Evaporative condenser: Fan and water pump.
Larger surface area of condenser contributes to lowering the condensing temperature closer to the temperature of cooling medium. This results in lower energy consumption.
Heat transfer of all condenser types reduces, if these are contaminated/ fouled
Air-cooled condenser space between fins should be free of debris, dirt and be in a clean condition always
in water-cooled condensers, tubes should be free of fouling by dirt, scale, corrosion etc.
non-condensable gas, air in system increase the condensing pressures and lower the efficiency.
Condensing pressure should be allowed to float with ambient temperature to take advantage of the lower ambient temperature at night and in winter time. However, this results in drastic drop in pressure ratio, which can cause system problems with commonly used expansion valves. Under such a condition, an electronic expansion valve or balanced port types are suggested. Or alternatively, raise the condensing pressure to maintain a healthy pressure ratio to operate effectively the selected expansion valve.
Expansion devices efficiency issues
Capillary tube / orifice plates : In refrigeration systems , refrigerant quantity circulated in an evaporator is critical for system capacity & efficiency. If a capillary tube is damaged or partly blocked, it will control the system correctly and the efficiency will reduce.
TEX Valve: Superheat setting has a significant effect on efficiency and system reliability
Low super heat will return liquid refrigerant to compressor leading to compressor breakdown
High super heat will reduce the system efficiency
TEX Valves do not control well over a wide range of pressure difference across it.
Issues related to refrigerant
Types of refrigerant can affect the efficiency of a system by about 10%
The relative performance & efficiency of a refrigerant is affected by the type of compressor and operating conditions
Charge of refrigerant should be optimum
Systems should be free of leaks
Contamination of refrigerant with air,N2 will result in lower system efficiency.
Common efficiency issues
The following golden rules apply for all systems:
The design & selection of condensers: These should be sized to maintain practically lowest discharge pressures
Evaporators: Should be sized such as to maintain practically highest effective evaporation temperatures
Suction line should be insulated
System should be charged with optimum quantity of refrigerant and system should be leak free
Defrost evaporator as & when necessary.
Over and above, a few simple things as mentioned below will contribute to saving the energy cost substantially.
Replace redundant old technology power consuming compressors, inefficient condensers and evaporators
Electrical power wiring too is a culprit example; loose wire connections, partly burnt- out cable/loose lugs, un-balanced voltage, switch gear contacts with a carbon layer, dust in the switch gear box, undersized cables etc. These issues contribute to the power leakage and entail extra energy consumption and results in additional energy cost.
Avoid frequent start stops of the plant since locked-rotor current would weaken motor windings.
Implement preventive plant maintenance practices
Replace/rebuild old cooling water treatment units & in-efficient cooling towers
Only trained manpower should operate the plant machinery
Limit outsourcing of technical manpower to only non-essential items
Install variable frequency drive units preferably on refrigeration compressors since it typically consumes about 50 to 60% of the total refrigeration system power
Carryout regular leak check of system & correct it, if found
Maintain cooling tower, condenser since condenser is a major source of energy leak. The compressor energy consumption rises rapidly concurrently with increase in head pressure
Monitor de-frost cycles in case of low temp cold storage
Avoid frequent door openings and monitor insulation of refrigeration piping for any damages/leakage
In AC plants, control infiltration load, if that is resulting in energy consumption, look for gasket leaks on ducting, slipping belts of AHU fan, check for cooling tower nozzles and fan for proper function, maintain tower hygiene.
The above suggestions are indicative, and individual system will need appropriate maintenance checks and schedule.
