The aim of any efficiency drive is obviously to deliver the biggest savings for the least cost and effort.
In relation to cooling plant, a number of relatively simple measures, each yielding a few percentage points in energy use, can combine to make a very significant impact on the energy bill.
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Reduce the condensing temperatures and increase refrigerant liquid sub cooling
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The number one tip for saving money on cooling costs is to reduce the condensing temperatures and increase refrigerant liquid sub cooling. For every 2deg C reduction in condensing temperature energy consumption is reduced by approximately 3per cent. On some applications it may be possible to cut condensing temperature by as much as 10deg C, which translates into a 15 per cent saving in the energy bill. With the average UK temperature in the region of 12 deg.C this is easily achieved.
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Make sure the refrigeration evaporating temperature or chilled water temperature is as high as possible
The second key advice is make sure the refrigeration evaporating temperature or chilled water temperature is as high as possible, subject to meeting the cooling needs of the application. For example - A 2 deg C increase can result in an increase in energy efficiency of around 5%. This can represent many thousands of pounds off the power bill.
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An undercharged refrigeration system can result in a significant loss of capacity
An undercharged refrigeration system can result in a significant loss of capacity. In some cases this can be as much as 15%. It is therefore essential to ensure that the charge is optimised and sight glasses/liquid lines kept full.
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Expansion valves should be carefully adjusted
Expansion valves should be carefully adjusted so that just the right amount of refrigerant is flowing through the evaporator. Too little refrigerant flow starves the evaporator resulting in a loss of cooling capacity. Too much refrigerant will result in flooding back which could damage the compressor so take care!
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Check regularly for blocked or obstructed airways at air cooled condensers
Check regularly for blocked or obstructed airways at air cooled condensers. Even partially obstructed units suffer a double penalty due to wasted fan power and the increased work required of the compressor. Accumulated autumn leaves, weeds and crisp packets come with a heavy energy penalty. Remove them!
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Condenser air recirculation is another energy sapping situation
Condenser air recirculation is another energy sapping situation. It’s easy to check if recirculation is evident by measuring air temperatures on to the condenser(s) and comparing them with the local ambient temperature. If the air on condenser temperature is more than the ambient temperature then air recirculation is more than likely occurring. Consider ways in which to get the condenser air discharge clear away from the equipment. This could be done by using a simple sheet metal air baffle or hood on top of the condenser framework. Condenser air recirculation during peak summertime ambient temperatures not only saps energy can also result in catastrophic failure of the plant just when you need it!
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Improved logic control can ensure compressor run time is minimised
On the controls side, improved logic control can ensure compressor run time is minimised. In the case of multiple fixed speed compressor systems it is important to switch compressors on and off in the right way so as not to cause short cycling. On variable speed compressor systems it is often more advantageous to operate all compressors at lower speed to benefit from improved isentropic efficiency.
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Hygiene on the other side of the system is equally important
Hygiene on the other side of the system is equally important. A partially blocked air conditioning evaporator again wastes fan power, and demands higher compressor pumping energy to deliver the required conditions. As well as compromised efficiency, blocked or dirty heat exchangers may result in inadequate duty output with resultant problems for building occupants or the process application being cooled by the plant.
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Ensure that water flows through evaporators and condensers are in accordance with specification
Where chillers are installed it is important to ensure that water flows through evaporators and condensers are in accordance with specification. If water flows are too high this puts more than necessary pressure on the pump(s) with a corresponding increase in power. In extreme cases tube erosion and or excessive water turbulence can result in premature failure of the heat exchanger . Closing down the DR valve will not achieve any power savings. Reduce the speed of the pump(s) instead.
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The Final Tip
Moving up the scale in terms of cost and time, it may be worth investigating the options for improved speed control for fans and/or compressors. Moving to inverter control, in the case of plant without it, can make a big difference to power use. Where plant is equipped with standard “phase chop” inverters, it is worth considering switching to frequency inverters, which deliver better control and offers improved energy savings over the operating range.
All of these measures apply to general air conditioning or heat pump systems, whether air- or water cooled. Cautionary note – adjustments to air conditioning equipment as stated should only be carried out by fully qualified refrigeration engineers.
The benefits of optimised condensing and evaporating temperatures
Simply optimising condensing and evaporating temps can result in significant energy savings in UK conditions, especially where there is a year-round load. Example: Many air-cooled chillers over five years old will have a refrigerant condensing temp set point of around 45 deg C, with a chilled water set point of 6 deg C. In most cases, the condensing temp set point can safely be reduced to 35 deg C and the chilled water temp can be raised to 7.5 - or even 8 deg C. Overall energy saving resulting from this would be in excess of 10 per cent With a typical building operating cost of around £35.000, there is therefore a potential saving of £3,500 just for making some simple adjustments!