pressure regulator purpose

[coldkeeper1]

The other day I listen at a technical fair to an interesting discussión between two transport refrigeration mechanics concerning the purpose of a pressure regulating valve supposely located in between the accumulator tank and the compressor, in several models of Thermo King units for mid-sized trucks (CB, maybe...I don´t know). They asked me to serve as a judge, but the truth is that both explanations sounded completely reasonable. and I didn´t know what to say. One tech assures that the purpose of this valve is that of keeping a more or less constant pressure / temperature in the evaporator, regardless of the suction presure which itself varies with the rpms of the truck engine. The other fellow indicates that the true function of this valve is to limit the suction pressure AT THE COMPRESSOR, protecting it against sudden high evaporator pressures due to abnormally high loads such as those which occur after defrost or during initial box pull down. Which one is correct? I already looked like an ignorant, but I hope you can help me out to look better the next time I´m asked to judge. Thank you!!

[Will]

The answer is B, the second guy.

(Edit; Hint, its named 'throttling valve' for a reason.)

[absolute-zero]

The other day I listen at a technical fair to an interesting discussión between two transport refrigeration mechanics concerning the purpose of a pressure regulating valve supposely located in between the accumulator tank and the compressor, in several models of Thermo King units for mid-sized trucks (CB, maybe...I don´t know). They asked me to serve as a judge, but the truth is that both explanations sounded completely reasonable. and I didn´t know what to say. One tech assures that the purpose of this valve is that of keeping a more or less constant pressure / temperature in the evaporator, regardless of the suction presure which itself varies with the rpms of the truck engine. The other fellow indicates that the true function of this valve is to limit the suction pressure AT THE COMPRESSOR, protecting it against sudden high evaporator pressures due to abnormally high loads such as those which occur after defrost or during initial box pull down. Which one is correct? I already looked like an ignorant, but I hope you can help me out to look better the next time I´m asked to judge. Thank you!!

Technician b is correct, as CPR valve is needed to regulate High pressure gas back to the compressor during heat defrost cycles. Technician A is refering to an EPR system evaportaor pressure regulator. this regulator is found in some non invertable multitemps and serves as setpoint based on pressure temperature relationship to control box temperatures at desired setpoint. The Epr would also maybe considered as a orit 10 valve typically found in Thermo King A/C system. This valve allows the evaportaor to stabalize at a given pressure allowing liquid from evaporator exiting to become vaporized by allowing High discharge pressur gas to be metered in. This will temper the evap coil during reheat mode and allow precise temperature managemnt of the transit bus or coach. I hope this clears up your question.

[coldkeeper1]

Thank you very much Will and Absolute-zero for your advice. I´m not very involved in transport refrigeration, but this topic really got my attention. Correct me if I´m wrong, but with your information I was able to investigate that the proper setting for this regulator is around 18 psi (124 kPa) for both R-134 and 404a.

[TDI]

normally there is no need of a throttle valve (edit: because the pressure on the suction line is regulated by the exansion valve)

TK use them especially in heat mode to save the comp from high pressure


hope its understandable

[absolute-zero]

normally there is no need of a throttle valve

TK use them especially in heat mode to save the comp from high pressure

I will back up this statement with you're absolutly correct. However units with electric standby will need some type of way to control labor and load on electric motor especially during initial pull down in high ambient conditions regardless of mode of operation.

Second note on the subject is, CB units are not equipped witha throttle valve but rather a CPR compressor suction pressure regulator valve. Although the vehicle powered units do not use hot gas heat as primary method of maintaining heat temperature inside cargo area, but rather heat is taken from the vehicles engine coolant system with optional heater coil, coolant solenoid valve, and electric boost pump. The hot gas circuit is there for defrost and only defrost mode.

CpR valve settings have differnt range setting based on unit model and if the unit is equipped with electric standby, but 18 psi setting for a CB unit sounds about right.

Regards A-Z

[TDI]

oh my god
heating with hot water...

this aera is over in europe the only unit i know was the V250/280 they could have been ordered with hot water heating but thank god no one did it
todays direct drive unit only heat with hotgas

so you can order a V300 or 500
with or without heating 30 with and 10 without

and also with standby and heatin (50) or standby without heating (30)

[absolute-zero]

oh my god
heating with hot water...

this aera is over in europe the only unit i know was the V250/280 they could have been ordered with hot water heating but thank god no one did it
todays direct drive unit only heat with hotgas

so you can order a V300 or 500
with or without heating 30 with and 10 without

and also with standby and heatin (50) or standby without heating (30)

You would order a V-200 30 model with a optional add on heat option. Or you can order the 50 model that has Electric standby, with or with out optional heat option.

Electric standby with heat option would have electric heat.

Regards A-Z

[Will]

Throttling valve settings for 404A can be between 13-29 PSI depending on unit.

I don't recall any 'Throttling valve/134A' combo's, not saying there isn't, just don't recall any.

[absolute-zero]

Throttling valve settings for 404A can be between 13-29 PSI depending on unit.

I don't recall any 'Throttling valve/134A' combo's, not saying there isn't, just don't recall any.

Will,
Your memory serves you correctly,there are no combos. either it has throttle valve, or CPR, but not both.

[Will]

I've seen plenty of electronic throttling valve and CPR valve combos. Albeit they were all shipping containers.
But regardless, that wasn't what I was talking about.

I meant "134a and throttling valve combos". Have you seen any of those ?

(( the OP is referring to "throttling valves", not CPR valves. ))

[Will]

What "CB Units" you all talking about ?

[absolute-zero]

Throttling valve settings for 404A can be between 13-29 PSI depending on unit.

I don't recall any 'Throttling valve/134A' combo's, not saying there isn't, just don't recall any.

Yes if the system is using hot gas heat regardless of refrigerant it either uses a TXV with MOP or throttle valve or CPR. I have seen many CBci2 HFC 134a systems with CPRs.

As far as container go I have seen DPR valves, CPR valves, suction stepper motor valves, KVQ valves, electronic expansion valves, and suction mod valves with MOP. Containers Have more gizmos and refrigerant valves out of any system I know of in transport.

[absolute-zero]

What "CB Units" you all talking about ?

Thermo King City Boss.

[o5g]

oh my god
heating with hot water...

this aera is over in europe the only unit i know was the V250/280 they could have been ordered with hot water heating but thank god no one did it
todays direct drive unit only heat with hotgas

so you can order a V300 or 500
with or without heating 30 with and 10 without

and also with standby and heatin (50) or standby without heating (30)

With out hot water heat in transport mode and electric in standby mode there is no way that those small compressors will heat in -40 winter. That is why we use the hot coolant and electric heaters in the small reefers.

[suny]

Dear koldkeeper

The second technician is correct. In all my transporter freezer trucks the suction pressure regulator valves have been installed. This is prevent suction pressure increase, due to hot gas defrost, temp changes in the cool chamber. If glycol plate freezers are installed, the back pressure at the start of the refrigeration cycle is very high and can cause damage to the compressor if the CPR valve is not installed.

[gravy258]

Interesting, on a Maxima 1200 theres a CPR bypass valve, to increase suction pressure in the heat cycle, hence increasing the heat capacity.

The CPR valve and throttling valve need to be set as per the manufacturers spec's for the unit in question. Capacity is reduced with each PSI drop in pressure.
Compressor protection and load are the reasons for a CPR valve, and to stop slugging of liquid back to the compressor, as this will destroy the compressor.
Over filling the compressor with oil also lowers the suction pressure.