How do Chiller Energy Performance vary at Part Loading Conditions

What happens to my chiller's energy consumption at part loading conditions when my chilled water supply temperature is constant?? 

People are keen to know how the energy consumption of their chiller is going to vary at part load conditions and even thinks of deploying a VFD to encash the savings as stated by affinity laws. But it's not always true that chiller consumes relatively lesser energy consumption at part load conditions, instead, it depends on various factors like the type of chiller, type of capacity control employed, method of the cooling tower fan modulation, ambient wet bulb.    

We in this article are going to give few insights on how chillers energy performance varies at part load conditions keeping in mind all related factors associated.

Performance of a chiller will be generally expressed in terms of kW/TR where it is directly proportional to the temperature difference between condenser water(Tc) and chilled water (Tch).

Therefore kW or power input is proportional to TR(Capacity) X ( Tc - Tch )

So in Ideal case power is directly proportional to capacity and Lift (considering constant condenser entering water temperature)

So it is evident that at a constant condenser entering water temperature when a load is reduced on a chiller there will be a reduction in kW/TR value since the leaving condenser water temperature also reduces with the load. (Reduced Lift)

But this holds good only in the ideal case. In actual conditions for reducing the capacity of a compressor either mechanical unloading has to happen or variable frequency drives has to be employed and the scenario changes in these cases.

For a centrifugal fixed speed machine at a constant condenser water temperature till certain break-even point, the kW/TR reduction follows ideal case and after that, it starts increasing because input power also varies in inverse proportion to the efficiency of a compressor too. Since at part loads efficiency point shifts to lower ends in a compressor this effect results in a net negative effect on total kW/TR.

So for 40 % load generally for a fixed speed machine and at higher condenser water temperatures around 85 degrees F, there will be an increase in kW/TR of a chiller. But in case if condenser water is able to cool further down and given then there will be a relative decrement in kW/TR consumption of chiller alone but on a whole ( kW/TR for chiller + kW/TR for CT Fan ) may increase/decrease - Refer Cooling Tower Optimum Control Strategy for more details on the same.  

And next, if we take a variable speed chiller for the same criteria then also the curve is almost similar at above 80 Degree F. There will be a very slight reduction in SEC at 90 to 100% modulation range but there will be an increment at very low loads of 40%. But if we could supply condenser water temperatures below 55 Deg F then there may be a little opportunity for a relative decrement in SEC at 40% load compared to 100% loading.

And finally, an important point to note here is when a comparison is made between SEC of a fixed speed chiller and Variable speed chiller ( 900 TR and 44 Deg F Chilled water leaving temperature) at 40 % load and entering condenser water temperatures above 80 Deg F fixed speed chiller SEC will better than that of a variable speed chiller ( As the losses due to VFD and compressor operating Efficiency dominates the effect of power reduction due to reduced capacity).

There will be a benefit in improved SEC at 40% loading for a VFD operated chiller than a fixed speed chiller only and when the entering condenser water temperatures are lower than 75 Deg F.

For detailed patterns of SEC variations with loading and condenser water temperatures, simulation has to be done on a case to case basis.

Hence before implementing VFD for getting benefit out of a low load condition, various points like power consumption reduction in the chiller at an available condenser water temperature on-site, the relative increase in CT Fan power and net power input to entire chiller system have to be considered.

*Picture Credits to Trane Engineer Newsletter Document