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rproper1
24-08-2009, 05:03 PM
Hi, I am new to this sight. I am looking forward to chating with some of you about my question.
The question is about having a solid liquid to the TXV.
I have two 500 ton systems that cool a ware house to about 60 degrees. They use R22. Each system has two recip compressors that stage in thirds. The system is still under warentee, so the contractor has controle of it still. We are located in Phoenix AZ. The contractor believes only enough refrigerant is needed to keep the liquid collumn solid at a steady state of running after a few minutes. The compressors cycle 15 minutes on and then 15 minutes off. The receiver fills from the bottom and this is were the liquid seal is maintained to the liquid line. When the on cycle starts, the liquid will empty out of the receiver and then the Liquid line will empty out of course, for one to two minutes before the receiver has enough refrigerant to make its seal and suply enough liquid to the TXVs. There are 30 evaps on each system, with soleniods on each. The contractor believes the system is more efficient running this way because there is less refrigerant to cool, but I believe it needs more refrigerant to keep the TXVs loaded so that the compressors are not just spinning waiting for the system to come into its steady state condition.
The company is a major player in the Phoenix valley here for many years, so I am sure they know what they are doing; but I asked for documantation of there theary, and was told no. Can anyone help me on this?
Thanks.

US Iceman
24-08-2009, 06:56 PM
Sounds like someone is trying skimp on the refrigerant charge.

The liquid line has to be full any time an evaporator is calling for cooling. It's that simple. Ideally, the receiver would have enough liquid in it to maintain the liquid seal when the system is running under full load conditions.

If the liquid line is installed vertically any distance (liquid refrigerant flowing up) it should also be subcooled, which means the liquid line would then have to be insulated too.

sterl
24-08-2009, 09:20 PM
This arrangement include regulators for head pressure control? Is there enough internal heat generation in the warehouse to create a refrig load when the outdoors is at a significantly lower temperature?

With that many evaps and that much staging on the compressors, the only reason for compressors to be short cycling by anything like design intent would involve maintaining oil return via suction risers....Do the machines unload before they cycle off? Are there liquid solenoids on each of the evaps? How many temperature sensors do you have?

acnerd
24-08-2009, 09:36 PM
If the liquid line is installed vertically any distance (liquid refrigerant flowing up) it should also be subcooled, which means the liquid line would then have to be insulated too.

Probably not the right place to ask, but I don't understand the above statement. I understand subcooling, but would have thought the liquid would cool off more in a bare copper pipe? (Depending on temp. around the pipe that is)
Thanks.

US Iceman
25-08-2009, 12:04 AM
Probably not the right place to ask, but I don't understand the above statement. I understand subcooling, but would have thought the liquid would cool off more in a bare copper pipe? (Depending on temp. around the pipe that is)
Thanks.

Depends on the liquid refrigerant temperature and the air temperature. If the condenser is air-cooled the refrigerant is still hotter than the surrounding air temperature, therefore the liquid line is rejecting heat. If the liquid line is colder than the ambient air the liquid line is absorbing heat, which means lost subcooling (if there was subcooling present).

When liquid flows up you have a static pressure penalty. This changes the pressure/temperature relationship. A liquid riser up a story building is sufficient to cause flash gas in the liquid line and some lost cooling capacity.

Sridhar1312
25-08-2009, 01:46 PM
Refrigerant in Liquid line need to be full and the whole system is sealed in otherwords if suffient regrigerant is not there the refrigerant will flash even before reaching TXV.
Best way is to check the sub cooling and superheat to ensure correct refrigerant charge is maintained, which will also ensure the rfefrigearnt is full in the liquid line till TXV

acnerd
25-08-2009, 05:02 PM
Depends on the liquid refrigerant temperature and the air temperature. If the condenser is air-cooled the refrigerant is still hotter than the surrounding air temperature, therefore the liquid line is rejecting heat. If the liquid line is colder than the ambient air the liquid line is absorbing heat, which means lost subcooling (if there was subcooling present).

When liquid flows up you have a static pressure penalty. This changes the pressure/temperature relationship. A liquid riser up a story building is sufficient to cause flash gas in the liquid line and some lost cooling capacity.

Thanks Iceman, very well explained. So pumping liquid refrigerant straight up, similar to water, can result in a pressure drop higher up .... you don't want to lower the pressure until the metering device; hence flash gas problems.

US Iceman
25-08-2009, 07:16 PM
Thanks Iceman, very well explained. So pumping liquid refrigerant straight up, similar to water, can result in a pressure drop higher up .... you don't want to lower the pressure until the metering device; hence flash gas problems.

Roger that! If the liquid starts to flash off you have less liquid available for doing useful cooling, hence the lost cooling effect. If the liquid supply gets low (pipe being emptied) as it feeds the evaporators(s) the suction pressure will start to decrease.

What it comes down to is: Any liquid line flowing up needs subcooling. And if you provide subcooling, the line should be insulated to keep from absorbing heat into the liquid line.

rproper1
26-08-2009, 03:20 PM
This arrangement include regulators for head pressure control? Is there enough internal heat generation in the warehouse to create a refrig load when the outdoors is at a significantly lower temperature?

With that many evaps and that much staging on the compressors, the only reason for compressors to be short cycling by anything like design intent would involve maintaining oil return via suction risers....Do the machines unload before they cycle off? Are there liquid solenoids on each of the evaps? How many temperature sensors do you have?
First question, watercooled towers with fan cycle. Second question, indoor temp 65, out door temp. 80 at night, 100 pluss in the day. Third question, yes they unload. and last, solenoids on each evap with sensors on each.

RANGER1
27-08-2009, 11:17 AM
Sounds like you have very little load in your rooms at all !
Is the warehouse being used with product in it and what is product ?

mad fridgie
27-08-2009, 11:36 AM
It sounds as if you are getting plenty of refrigerant mass through the evaps regardless of the charge. A longer term problem is the damage which being caused to the TXV seat (drawn), this could cause poor superheat control, and potentially compressor flooding, reducing the life of the compressor or even destroying it!