C.p.r. Valve

[stevemartial]

hi guys, just come across a refrigerated vehicle with a c.p.r. valve fitted with a hot gas defrost system.
my question is -what does a c.p.r. valve actually do?
(i know it's a pressure regulator) but why do some units have them and some do not?

[chillyblue]

Hi

It is there to protect the compressor from high loads after the defrost, after a defrost the suction pressure can rise dramatically, this valve will reduce the flow back to the compressor untill the pressure is at a resonable level for which the compressor is designed to handle. the valve can be set using a amp meter to measure the compressor amps, the valve should be set to close when the full load amps of the compressor is reached, any pressure below this and the valve will be able to open fully.

most systems don't require one as they often run with a stable and constant load.

It can also be acheived by using TEV's with a MOP setting.

CB

[TXiceman]

CPR stands for crankcase pressure regulator. It basically controls pressure down stream of the valve (compressor side).

It is used to keep excessively high suction pressures from the compressor during operating cycles such as hotgas defrost of hot plant pull down.

Ken

[US Iceman]

...the valve can be set using a amp meter to measure the Compressor amps, the valve should be set to close when the full load amps of the Compressor is reached,

I would offer a word of caution about this procedure. If the full load amps are used to set the CPR valve, you could run the risk of allowing the discharge pressure to get too high.

The condenser was probably selected for the design operating conditions, which includes the suction pressure. As the suction pressure increases the total heat of rejection also increases.

Using only the compressor amps may create another problem.

I would recommend setting the CPR only slightly higher than the design suction pressure to prevent from overloading the condenser.

[Will]

On the flip side, many of the Carrier containers I work on have a D.P.R. valve. (discharge pressure regulator)

Why have one ???

[TXiceman]

As note by US Iceman, it is dangerous to set using only high amps. Some compressors have an upper suction pressure limit (such as a screw compressor) and you need to look at this. On a recip you can also add suction pressure switches to keep cylinders out of service during high suction which will prevent motor and condenser overload.

The newer generation of industrial screw compressors often use micro panels to limit suction pressure loading, and also limit loading on high amps and high discharge pressure.

In the case of a screw compressor, excessive suction can lead to over pressure in the rotor cavity prior to opening to the discharge port. This high pressure will lead to bearing overload and premature bearing failure.

As for a DPR, I have used them on systems where I had to maintain a pressure due to oil feed problems at low head, sufficient pressure to feed the liquid to evaporators and also to have sufficient pressure for hotgas defrost.

Ken

[Samarjit Sen]

I have used the Crankcase Pressure Regulator valves in a number of my low temperature application projects. It does just what Ken said yesterday.

I have noticed that while it keeps the suction pressure down during the Hot Gas Defrost, the working of the system is improved at other times.

[downunderman30]

set the cpr of a carrier unit, when the unit is on a heat or defrost mode, depending on the unit will determin the presssure it should be set.

[Pooh]

On most systems I have worked on the CPR has been fitted to protect the shaft seal after defrost on small open type comps. Usual setting is to the saturated pressure at the highest expected evaporating temp after defrost against the room temp.

Dont like the idea of using an ameter at all.

Ian

[chillyblue]

On most systems I have worked on the CPR has been fitted to protect the shaft seal after defrost on small open type comps. Usual setting is to the saturated pressure at the highest expected evaporating temp after defrost against the room temp.

Dont like the idea of using an ameter at all.

Ian

Hi

Suprises me that nobody has ever seen the amps rise due to high load, i've set loads of these valves using a ampmeter to monitor the compressor amps, and as iceman rightly said, which i forgot to mention, head pressure as well.

Try it you might like it!! and it may save people setting overloads far to high because it keeps tripping
Could save your compressor too

Cheers CB

[geoffthefridgem]

Hi Steve on our units (hubbards) we fit a cpr valve to stop the standby motor tripping the overload on start up especially after defrost as our units run very close to 13 amp as the norm.

[thermo prince]

Main function of thre c.p.r on OED off-engine drive transport units, <or throttling valve on larger self-powered diesel units> is to regulate the suction pressure back to the compressor, preventing excessive loads on ELECTRIC STANDBY operation when on heat/defrost cycles.
Nuisance shutdowns on OLR tripping etc is not desirable nor is excessive high discharge pressure (HPCO possibility) on heat /defrost.

However, these valves will throttle capacity on cooling too, affecting pulldown times or recovery after door openings, defrost and such.

So a good compromise setting value is specified by manufacturers, model by model during lab tests.

I would advise generally don't tamper with the settings or modify unless you are quite experienced and have good feel for the environment, ambient degC, work conditions etc of the equipment you may be asked to service.

On road operation < when 'road' compressor is driven off engine crank pulley vee-belt> or units without the electric standby, in theory the valve setting can be upped to the limit of what the condenser can properly " deal with" on the discharge side & what small swashplate compressors can tolerate. Small cc regular recips are more robust of course.

Some 2nd hand Japanese "Jimports" truck can be seen w/o cpr valve at all. Usually no standby on those.

[thermo prince]

Purpose of the DPR restrictor, discharge pressure reg, is to try improve the discharge vapour heat content by discharge press/ hence temp increase in very low ambient weather. Important to improve this for heat/defrost cycle times - at certain time and condition, some compressors of screw and scroll may not have the same degree of comression heat added as say piston type.

[REEFER-TEK]

hi guys, just come across a refrigerated vehicle with a c.p.r. valve fitted with a hot gas defrost system.
my question is -what does a c.p.r. valve actually do?
(i know it's a pressure regulator) but why do some units have them and some do not?

ON Thermo King truck units utilizing hot gas heat for heat and defrost it Limits refrigerant flow back to the compressor during heat defrost mode, This reduces engine load and or electrical current load on units equiped with Electric standby.

Most all units equiped with this valve have spec setting in there respective service manual.

Tk units also come equiped with a throttling valve as well, this valve works in the same fashion as the CPR. You will find this valve, on large capacity truch units and all trailer nose mount units.

Truck units utilizing open drive scroll compressors, you will find another regulator known as a discharge pressure regulator "DPR". This gives the unit improved heating as it regulates discharge pressure at 350 psi in heat and defrost. This also gives the compressor improved capacity due to increased back pressure on the scrolls.

CPR valves also can be found in evaporators but works more as evaporator pressure regulator, the setting on this valve dictates what the coil temp will run at specific to press temp relationship of refrigerant being used. and desiored temp control set point,

Example would be remote zone 2 temp compartment needs to maintain a temperature of say 35 degrees
and the unit is charged with R404a, we would set the pressure regulator for around a pressure of 50psi, of course most coil temps operate 10 to 15 degrees below box temp so there you may need to set it lower so the box will maintain the specific temp.

I hope this was of help.