REFRIGERANTS
Condenser efficiency - as for the evaporator, the efficiency is a more informative KPI than approach as it defines how much efficiency deviate compared to an ideal condenser operating at the same conditions.
Approach in condenser is a KPI affected by air flow, fouling and refrigerant charge. Efficiency and approach are affected by: Condenser fan controls and setpoint Air flow issues, air leaks broken fans/fan blades will increase approach Condenser fouling cause increased approach and decreased condenser efficiency Overcharge decrease performance (change subcool).
Sub cooling at a given operating condition and capacity will directly reflect refrigerant charge in many systems – where operating without a liquid receiver and has for example an expansion device controlling superheat or liquid level in evaporator – as the condenser acts as the buffer for the different refrigerant charges required in evaporator. Deviation in subcooling would also occur when pressure drop occur on high side.
Setpoint condenser fans. Far too many air-cooled chillers are still found to be operating at 40- 45°C year around. This is the low hanging fruit even if it requires understanding of chiller operation to be set as efficient as possible within compressor/chiller supplier envelopes. Optimising this setpoint often saves 20-25% on annual energy consumption in cool climates as central and northern Europe.
On/off fan control – to turn fans on/ off is the least efficient method of controlling minimum condensing pressure as it often cause the system to operate very inefficiently as when condenser fans turn off pressure decrease and cause warm liquid to flash in the liquid line before the expansion valve – this causes the expansion valve to open and evaporation to decrease, but as soon as the liquid has cooled down the valve is open too much and there is a risk of liquid flooding to compressor – so the expansion valve closes
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rapidly, capacity decreases and the fan turns off... and around it goes.
Compressor performance KPI Often the only indicators used to evaluate compressor is limits on discharge temperature and currents which are very crude as a compressor can be operating at very low performance without tripping alarms until you reach peak demand at extreme weather. At that time the compressor has often used excessive amount of energy for months or years and caused break down of oil and acid formation making repairs challenging and costly.
Compressor isentropic efficiency is the direct KPI proving that the compressor is maintaining design performance. When the decrease start it is time to take action not when motor protection or discharge limiter trip. For larger semi-hermetic and open drive compressor repairs can be done at fractions of the cost and for hermetic compressor a planned replacement before there is a burnout is drastically much better than an urgent call out to replace compressor when it fails – often at peak load when it is pushed over the edge.
System performance KPIs COP is a well-known KPI for performance but rarely correctly understood as the same unit could have COP 6 in the morning and COP 3 at lunch while still being as 'efficient'. COP changes with outdoor and chilled water temperature. COP has to be related to a specific operating condition to tell you anything at all. An efficient system should operate with a wide spread of COP because the only way of having constant COP is running at 'worst case' conditions at all times.
Cooling/heating capacity has the same issue as COP – it varies with conditions and further most plants never require 100% capacity as the design contains safety margins even at peak load. If sub efficiencies listed as KPIs in this article are good, then the design capacity will be reached when required.
System Efficiency Index (SEI), is a powerful overall KPI for benchmarking as it compares
T
he images above demonstrate a practical case recently executed by Birdsall Air Conditioning in the London area showing the visualisation KPIs in a dashboard. The top Dashboard visualises the need to improve condenser control of the chiller and the second the savings achieved with efficient EC fans and improved fan controls. The savings shown are achieved by replacing condenser fans with efficient EC-fans and optimising performance by adjusting condenser
the measured COP with the COP of a loss-free process at the same operating conditions (Carnot efficiency for reference temperatures instead of condensing and evaporation temperatures). It will indicate and through the sub efficiencies above identify any decrease of performance.
fan setpoint. The KPIs shown are for a 15°C outdoor temperature. Pre-optimisation, condenser effi- ciency is 44%, condenser approach is 25°C and COP is 3.81. Post optimisation, condenser efficiency has improved to 62% from the improved air
flow.The condens- er approach is 15°C and the COP has increased to 4.99
This results in an energy saving of more than 20% and significantly reduced compressor wear.
Informative KPI’s such as SEI and sub-efficiencies for the condenser, evaporator and compressor identify poor efficiency and visualises the causes when the deviation occurs, triggering a warning. They are relatively stable over a wide envelope and when there is an alert all information is available – and usually there is plenty of time before the system will trip.
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