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squidward
26-06-2009, 08:14 PM
Hello, I've got a air seperation plant with a Vilter 456XL chilling 780,000 cu.ft/hr @ 90 deg F. to around 40 deg.F. My target suction pressure is 65 PSI. The plant installed a pressure transmitter and controlling loading and unloading with the plant computer system. I have fixed several things already: evap level float controller new needle and seat, correct spring( it was calling for liquid 100% of the time) didn't flood the evap. due to low level in the receiver, so added *****(R-22), installed a new compressor, and have been struggling with a suction pressure that goes from 60 to 74 PSI in cycles.

Come to find out they put a timer on the setpoint to load and unload for-get this- 30 seconds! It would pull down at 100%, hit the setpoint of 64.5 PSI, and not unload til 60 PSI or so. Then it would unload to 66%, then 33% and turn on the hot gas! Then the pressure would shoot up, past the load setpoint of 65 PSI, start timing, and finally load, first at 66% and finally 100% By then of course the pressure is at 70 or more, all the liquid is at a full boil, and it would take (depending on how hot it is) a long time, or never,to come back down.

I got them to let me adjust the timers(2 sec. delay to load, 5 to unload) I cannot convince them that they need to get rid of the timers( they say they need a "deadband") and set up 4 different setpoints for the loading and unloading and another one for the hot gas bypass. This would control it and give it a pressure controlled deadband rather than a time based deadband between steps. They don't think it's a problem because it's run this way for years(until I came along and started fixing stuff) It's been blowing hot gas thru the liquid add line for years due to a low receiver!

Does anybody know where I can get some data to show these guys? They just don't believe me.

US Iceman
26-06-2009, 10:14 PM
Ideally and most often the hot gas bypass valve is not allowed to energize until the compressor has reached its minimum capacity (33% in your case). The compressor should load and unload on suction pressure for the 66% capacity solenoid (unless you have a different unloading configuration on the compressor. What you mention is the standard arrangement.

The unloader switches should be set to maintain a reasonable suction pressure within a narrow range. However, if the load from the chiller fluctuates the timers might offer a method of slowing down the response rate to capacity changes to a minimum.

Each one is a little different though.

PS. can you use some paragraphs please? A solid block of text is kinda hard to read.;)

squidward
27-06-2009, 05:22 PM
Sorry about the run-ons! Our

squidward
27-06-2009, 05:23 PM
Heat load is very constant, changes directly with ambient air.

US Iceman
27-06-2009, 07:04 PM
Heat load is very constant, changes directly with ambient air.

OK, if that's the case the suction pressure should be relatively constant also.

Part of the problem may be due to the low refrigerant charge. Under a higher load (without enough refrigerant in the air chiller) the suction pressure could drop and then the hot gas valve kick in.

You also need to find where the refrigerant went to that leaked out! This is important not only from the added cost but also EPA regulations and the phase out of R-22 next year...

squidward
27-06-2009, 10:01 PM
It was a cracked pipe nipple that caused the leak. Upon filling, discovered they've been wrongly estimating charge for years. Yes it is reported

RANGER1
27-06-2009, 10:30 PM
squidward ,
When you say hot gas bypass do you mean high pressure gas from liquid reciever into compressor suction line ?

If so you could also use a regulating valve ( crankcase pressure regulator ) .
On low loads it will modulate high pressure gas into suction line to keep pressures more smooth and constant .

The solonoid can be used as a back up with pressure switch in case modulating valve leaks etc .
If liquid level in evap not high enough or reciever level low ( blowing hp gas into evap ) suction pressures will also fluctuate wildly . Normally liq reciever level should be high enough to allow for varying loads , condensor fan cycling etc .

Looks like your on the right track anyway .

squidward
29-06-2009, 02:19 PM
Hey Thanks Ranger, Yeah we're gonna activate HG bypass and regulate gas in.

US Iceman
29-06-2009, 02:42 PM
...we're gonna activate HG bypass and regulate gas in.


I thought you said you were already using hot gas bypass. It should already have a pressure regulator in the hot gas bypass line.

Typically, these are provided with one of the two arrangements I have seen used for this type of system:


An outlet pressure regulator with an integral solenoid to lock-out the regulator operation, or
An outlet regulator without an integral solenoid, however in this case a separate hot gas solenoid would be used.

These are usually set-up as follows: the hot gas solenoid is energized when the last step of compressor capacity is reached (in your case 33%). Then at this point, the hot gas regulator begins to modulate when the suction pressure begins to decrease below the set point.

You should have somethings like this:


Compressor reaches 33% capacity.
Hot gas solenoid is energized
Evaporating pressure decreases as cooling load decreases
When the evaporating pressure falls below the deadband (usually 2-4 psig) below the set-point for the 33% solenoid the hot gas bypass regulator begins to open.
If the cooling load disappears completely the hot gas bypass regulator would be full open at that time.

On flooded air chillers the hot gas (from the bypass regulator) is normally introduced below the liquid level in the chiller. This provides desuperheating of the hot gas (if the hot gas is from the discharge line).

Has the hot gas bypass regulator been adjusted? It needs to be set also.

squidward
29-06-2009, 06:05 PM
Thanks, Yes it is a nice regulated HG valve- I am more concerned with having a timer in every step causing wide range of suction pressure. Because of time delay, it will go to 33% when 66% should be all it needs.