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laudrup
21-05-2013, 02:43 AM
Hi,

Increasing suction pressure is a common recommendation in energy efficiency guidelines published by various bodies.

I am looking for a good simple explanation on how this is typically done in practice. Specifically I am interested in the case of industrial refrigeration plants (either packaged R134a or R22 chillers or large ammonia plants) that chill glycol into a tank (glycol temp can be -8 to +4 deg C), which is then reticulated for process cooling.

I understand the basics of refrigeration but I currently know very little about the practicalities of refrigeration plant control systems.

Thanks in advance for your help.

Tesla
21-05-2013, 05:08 AM
Hi laudrup
to put it basically simply increase the chilled water temp setpoint. This would be a seasonal change to match loads when they are lower in the winter. Load matching by math and practical demonstration show when the load is closest to the source the efficiency will be the highest. By increasing the suction pressure or decreasing the discharge pressure will increase output over input function - this is best shown on a pressure enthalpy (H) chart.
Example an AHU will require chilled water around the range of 15 to 6 degC, on a hot 40deg day we might try for 6deg if we can get it to match the load, and a cool day of 15 deg we may only require 14 deg chilled water to match the load. You will need to understand the pressure enthalpy chart so check it out. You could save even more energy if office workers would dress accordingly for the day temps - like wear warm clothes when it is cold and wear a bikini or swim shorts on a hot day, not quite but ... also check the ashrae 65 comfort chart.

laudrup
21-05-2013, 05:32 AM
What does the chiller do when you change the chilled water setting though? Does it adjust the expansion valve or some other refrigerant valve?

Thanks,
Laudrup

Rob White
21-05-2013, 08:50 AM
.

Think of it in real simplistic terms.

Which component is the most expensive one to run on any system?
It will be the compressor or compressors (multiple).

Now the only reason why you have a compressor is to create a pressure
difference and that pressure difference is the bit that can be controlled.

If you work your compressor/s easy then the cost to run them is less than
running them hard. So you set your operating parameters to be as near to
the temp you want.

How you control that depends completely on the system and control strategy
in use.

If the purpose of the compressor is to create a pressure difference, then the
metering device/s, expansion valve/s or other pressure reduction system is
installed to create a pressure difference.

By controlling the speed of the compressor (or its pumping capacity), the amount
of restriction over the metering device and the temperature difference across the
condenser and evaporators, you can manage the running costs and keep them to
a minimum.

Rob

.

laudrup
21-05-2013, 10:56 AM
Thanks Tesla and Rob for your help. I understand that a higher suction pressure and lower discharge pressure mean that the compressor motor has to do less work so the COP is higher.

My question is if I had a refrigeration system and I wanted to raise the suction pressure, what actually would a technician or a control system typically do in practice to realise this.

For example, say I had an ammonia refrigeration plant that cooled a tank of glycol, with the refrigeration plant and pump that moves glycol from the tank through the evaporator and back into the glycol tank, started up by a control system when the glycol tank temperature reached -6 deg C and then stopped by the control system when the glycol temperature reached -8 deg C. If for some reason I no longer needed such cold glycol and therefore I set start and stop points of -3 and -1 deg C, respectively. What changes would a technician or a refrigeration plant control system need to be make to the refrigeration plant itself so that the suction pressure increases and the refrigeration plant therefore operates more efficiently. Or is this a completely automatic feature of any refrigeration cycle independent of any valve or adjustment, and when the glycol is warmer the suction pressure just gets higher by itself.

Thanks again,
Laudrup

Tesla
21-05-2013, 12:16 PM
To put it simply for most systems it is automatic - when the temp of the water or glycol is higher so is the suction pressure and the efficiency. By increasing the set point higher the suction pressure increases. Increasing the suction pressure is the best method. Some systems waste energy on the discharge pressure as they rely on artificially increased pressure to return oil to the compressor - Screw compressors are the worst in this situation.

laudrup
21-05-2013, 02:20 PM
Thanks Tesla,
Just to confirm, would the suction pressure increase automatically irrespective of whether a capillary tube, automatic expansion valve, thermostatic expansion valve, float type expansion valve, or electronic expansion valve, were used?

Tesla
21-05-2013, 11:34 PM
Generally speaking except the electronic yes.