PDA

View Full Version : Energy savings







Daz B
25-08-2005, 03:36 PM
I am looking at energy saving oppertunities for industrial recipricating chillers.
The main area I am looking at is reduced head pressures.
I have looked at Liquid pressure amplification and also the use of Danfoss PHTQ EEV and associated pulse width modulation controller.
The main points I seem to be coming back to is, maintaining enough delta P to keep oil circulating, keeping an eye on sub cooling for flash gas before EEV / TRV and capacity of the valves at reduced delta P.

On my own steam I seem to have concluded that EEV is the cheaper option, but LPA may be a better option under air cooled condensors due to flash gas issues.

I have seen a document by York which compares all three together (TEV, EEV and LPA) and the savings from the EEV was similar to the LPA system.

Any experience / comments please.

US Iceman
25-08-2005, 04:26 PM
When you say industrial reciprocating chillers, are the compressors semi-hermetic or open drive (motor connected to compressor with belts or coupling)?

The two biggest items that go together are: head pressure reduction and effect of lower dP for expansion valves.

If the compressors has a direct drive pump right off of the compressor crankshaft (as most do) the net oil pressure (oil pump discharge pressure over crankcase pressure) should not be affected.

However, as it seems you are aware of the dP problem as it relates to the EEV and reduced dP (lower discharge pressure & fixed evaporating pressure) I won't go into that.

Most compressors will have a minimum pressure ratio requirement. A decent starting point is 2:1. If the EEV can modulate the liquid refrigerant and control superheat consistently at this pressure ratio, you should be OK.

Flash gas is a different thing. Flash gas is either caused by a pressure loss (friction or static) or heat gain into the liquid line.

If the liquid line has flash gas inside, I would investigate the cause. For example, if the liquid line travels upwards in a vertical direction, the difference in static pressures will cause some of the liquid to flash off.

Another example is if the liquid line travels through a high ambient temperature or is exposed to a temperature higher than the saturation temperature of the liquid. Heat gain into the liquid can cause flash gas.

Air-cooled condensers can have a lot of power input for the fans. Perhaps a variable frequency drive could be used to modulate the fan speed down. Even if you can get the fan speed down to 75% of the full speed, a decent savings can be found.

Some thoughts for you to consider.

An operation scheme would be to push the discharge pressure down to save energy and run the fan speeds lower to minimize energy use. It may be a case of using more fan energy to save compressor motor energy. The main thing to consider is a scheme which uses the minimum amount of energy.

Some last ideas.

Do you have off-peak power rates? High energy costs during the day and lower costs during the night hours? If there is some variability in your cooling loads (day to night), you may be able to use the evening hours to generate cold water for storage. Any operation you shift to the evening hours, results in a lower overall power charge. Load shifting and/or thermal storage??

If the chiller operates year round you could also use a water-side economizer. Use the outside low dry bulb temperature to cool the heat load. You need to design the system to prevent freezing (maybe glycol, PHE, or closed circuit fluid coolers) but it can be done.

Peter_1
26-08-2005, 07:02 AM
See also http://www.refrigeration-engineer.com/forums/showthread.php?p=25129#post25129

http://www.refrigeration-engineer.com/forums/showthread.php?t=1748&highlight=head

Daz B
27-08-2005, 10:19 AM
Thanks for the reply.
The system I am hoping to try this on consists of 5 semi hermetic units, largest power input is 90kW and running on R22.
These are water cooled condensers which are packaged units with short liquid lines, hense I think this will have the best oppertunity for EEV's (less liquid line flash gas issues). They chill a common glycol tank.
There is also a seperate air cooled unit with two common compressors again semi hermetic which I propose may be more suited to LPA (due to long liquid lines to the condenser outside the plant room) as the liquid pressure amplification will provide the sub cooling required.
Have you any experience of the Danfos PHTQ valves? They state they can operate under reduced head pressures OK.
For the energy saving side of things I seem to have come to the conclusion as a rule of thumb, (providing the EEV,s operate OK), long liquid lines use LPA (although 4 times the cost), short liquid lines more potential for EEV?

US Iceman
27-08-2005, 09:53 PM
I wanted to make a comment on the LPA system. The subcooling (provided by this system) is developed through the increase in liquid refrigerant pressure at constant liquid enthalpy. No additional capacity is created by the use of this device/system.

Since the enthalpy of the liquid is not affected, the net refrigeration effect (NRE) does not change.

Any apparent or perceived increase in capacity, when the LPA system is used, is caused by the delivery of 100% quality liquid to the expansion device. The "equivalent" subcooling generated with the LPA system only enures the liquid has no flash gas in the liquid line.

On air-cooled systems, the liquid refrigerant temperature is normally higher than the ambient air temperature. Since the liquid is at higher temperature, heat is rejected to the ambient. This could be stated as adding subcooling.

If the liquid lines are long and you have a lot of flash gas, there is either an excessive pressure drop due to static or friction losses, or, the liquid line is absorbing heat from some external source.

My general rule is insulate the liquid lines if the ambient temperature is higher than the liquid temperature. Or, leave the liquid line exposed if the ambient temperature is cooler.

For high friction or static losses you will need colder liquid, or something like the LPA, or change the piping to reduce the high friction losses. High static losses can only be overcome by a pressure increase or lowering of the liquid temperature.

I have not used the Danfoss valve you describe. However, I have used the Sporlan balanced port TXV's for the application of low discharge pressures. Have had very good results with these.

Hope this helps.