Due to increasing electricity costs I’m am under pressure to reduce operating costs for the site decribed below, any thoughts or ideas would be greatly appreciated.

I look after a cold storage site, three stores each has its own plant room. each plant is pretty much the same set up:

R22 pump circulated, flooded evaps

2 stage compound compressors with interstage cooling provided by TEV liquid injection and vapour return from subcooler/econimisers which are TEV feed,

hot gas defrosts (pressure regulators on evaps),

air cooled condensers,

no oil separators, oil is returned via a discharge line heat exchanger TEV feed,

PLC controlled.

Grenco installs 130, 195 & 288kw.

At present we run the plants with two 4 hour shut down periods per day, controlled by time clocks, the 4 hour periods are extended/reduced depending on ambient temps, time of year, store operations, etc etc (temps are very even across the stores during shut downs) Normally around -23/-24 when switched off and around -18/-19 when switched on.


We have thermostats/controllers on each evap and when a satisfied temp is reached on an evap the liquid line solenoid is closed and three of the four fans are switched off. I have tried in the past to operate using the stats but found I end up with one evap working and one compressor on 50% or compressors & evaps cutting in and out to frequently and this proved not to be as efficient as the operating method above, ie a higher kwh per day. I think this method of control is more for close control of temps?
At present these stats are set to -29 which is never reached.
The way I see it is that if you have a 90kw motor running a compressor at 50% then this has to be less efficient than running the compressor at 100%


I have Mitsubishi PLC’s controlling the plants with HMI’s which allow me to change defrost lengths, times, loading & unloading times for compressors, duty selection, and various other things, so we have good control over the plants with a lot of control options.


I am thinking of changing the control philosophy of the plant and using one thermostat per store/plant to switch plant on/off rather than evaps switching on & off regularly and compressors unloading & loading all the time. I think this way I could have the stats set to switch off the plant at -23 and back on at -19 and it may reduce running hours.


We have always maintained a discharge pressure of around 10 bar during normal running, this is increased to 11 during defrosts via a constant pressure regulator. I am considering running at a lower pressure during normal running but how low can I take it before it will start affecting the performance of the TEV’s involved in interstage cooling and oil return and will this actually achieve anything ?

Also does any one have any experience or comments on inverters for condenser fans this is another option we have been considering.

We already have a very good kwh per cubic meter ratio on this site compared with others within our company but I’m still under pressure to save and it’s getting ridiculous, some of our management (coldstore managers NOT engineers) have attended a conference last week and a presentation was given by supposedly an energy management consultant who said “in most cases the evaporators are over specced on the fans normally by 50%”
What a W?@&%R,

I now have received an e-mail from management telling us (not just my site, but all others in this region) to fill in a questionnaire about how many evaps & fans we have in each store and they (remember these guys no nothing about refrigeration engineering)
will tell us how many fans/evaps we have to run.

This is what they suggested:

If you have a store with two evaps with four fans on each can you turn off one evap or run both evaps but only use two fans per evap

Then, This one made me laugh,

Fit time clocks to the fans so at weekends or times of minimal door openings we can switch off all the fans.

So I’m now expected to run my plants at weekends with no evap fans! This should be interesting.


Sorry about the long post but I wanted to give as much info as possible,

Any criticism, ideas, advice would be greatly appreciated.