mid-size refrigerated truck Thermo King unit question

[coldkeeper1]

I´m undertaking my first jobs as a Transport refrigeration mechanic. One of them deals with the repairing of a THERMO KING CB MAX unit. It has both an engine driven compressor as well as an electrically driven unit. Client reports an excessive long temperature pull-down time. He suggests, from his previous experiences with a similar truck, the replacement of a larger capacity compressor in order to improve performance. I´d been trained to listen up to any equipment´s user suggestion, but this one doesn´t make sense. In order to boost performance with a larger compressor, isn´t it true that a larger capacity condenser and fan must be included in order to handle the increased high side heat load?
I´ve noticed that the compressor´s refrigerant liquid injection cooling sensor was disconnected by the previous tech who worked on this unit. At what pressure should this sensor activate the injection control solenoid in a R-404A system? Thank you.

[abbsnowman]

First of all I suggest you get in the back of the truck and close the door. Do you see daylight anywere? If so replace seals.
Second, you can change the compressor but why? If it worked before, likely there are other issues such as mentioned above.
As far as the liquid injection sensor. It is not for pressure it is tempurature. The sensor is to be located as close as possible to the compressor on the discharge line and wrapped to ensure proper distcharge temp. reading. If temp. gets to set amount, it allows liquid to enter the suction side of the compressor and thus, cooling the compressor.
Hope this helps!:)
Abbs

[coldkeeper1]

Thank you abbsnowman for your reply.

Door gaskets seem to be all right.
I found and checked with a multimeter the temperature sensor of the liquid injection compressor cooling system and seems to be open. That´s why it was eliminated previously. I have spare sensors for other units around, but none seems to be the exact replacement. Any idea of the temperature / resistance relationship for this particular TK sensor?
Thank you.

[abbsnowman]

Although I do agree that that sensor should be replaced, this particular scenario will not cause poor cooling. If valve was stuck open it would but not shut. The liquid injection is used to cool down the compressor. I would not use any sensor aside from factory as if it is not exactly acurate, you risk sending liquid to the compressor at the wrong time and causing compressor failure.
Can you tell me what the pressures are and is ther ball float in the reciever tank? (Located on the driverside of unit.)
Also, often people forget that boxes get saturated with moisture over time. If they don't believe me I have them weigh the truck and compair it to when it was new. Pretty hard to cool a bag of water!
Anyways, if you could tell me the pressures and ball float. I would be happy to help you further.
I have alot of experiance with these and offer my help to anyone who needs it.;)

[SKOOBY]

How big is the body/box and how many times do they open the door over a given period on deliverys.
Have you checked the delta t air on and off the evap.If this is around 7degrees.The capacity is there

[abbsnowman]

How big is the body/box and how many times do they open the door over a given period on deliverys.
Have you checked the delta t air on and off the evap.If this is around 7degrees.The capacity is there
Good call. We use the 8-12 theory.
Did ya vote on the fox scooby?

[SKOOBY]

No foxes here Mores the pity

[absolute-zero]

Liquid Injection control switch: Opens 200 ± 5 F (93 ± 3 C) and Closes 230 ± 5 F (110 ± 3 C)I have found, and as well TK recommends; if temps are to be maintained in a fresh temperature range: such as 25F, you should be using R-134a, and if the product is to be maintained at below those temps, then use the R-404a. The unit will work more effieciently. Sizing up bigger swash plate compressors, will nor require you to increase condensor size, as it will not cause that much of an imbalance in the system. The rules you should follow when selecting a compressor are mostly considered around engine rpm speed. Some of the buigger swash plates do well in diesel but not in a gasoline engine, because gasoline engine can double engine rpms vs. a diesel engine.If compressor or any compressor expeirences over-reving, it will ruduce compressor life and will definetly fail.Hope this helps.

[coldkeeper1]

Thank you abbsnowman, skooby and absolute for your very informative replies. My supervisor decided to change the compressor anyway. I´m currently installing a larger unit. I also asked for a TXV spare, and the liquid injection temperature sensor is coming by courier.
When I worked at a domestic car air conditioning repair shop, I was instructed to distribute the new oil charge among the system components: one-half to the compressor, most of the remaining half to the evaporator and accumulator, and I few drops to the filter/drier. Does this method apply to transport refrigeration units? Also, as we deal with long refrigeration hoses, is the 30 ml /meter of hose additional oil charge rule good when accounting for this?

[absolute-zero]

Thank you abbsnowman, skooby and absolute for your very informative replies. My supervisor decided to change the compressor anyway. I´m currently installing a larger unit. I also asked for a TXV spare, and the liquid injection temperature sensor is coming by courier.
When I worked at a domestic car air conditioning repair shop, I was instructed to distribute the new oil charge among the system components: one-half to the compressor, most of the remaining half to the evaporator and accumulator, and I few drops to the filter/drier. Does this method apply to transport refrigeration units? Also, as we deal with long refrigeration hoses, is the 30 ml /meter of hose additional oil charge rule good when accounting for this?

Refer to the yellow maintenance manual of the specific unit you are working on for the recommened oil and the amount. Be cautouse to not use an automotive PAG oil as this is not optimal oil for refrigeration. After you have charged the new compressor and optional electric stanby compressor with recommended oil type and amount, you the will add the remainder amount into the reciever tank.Hope this helps.Regards A-Z

[abbsnowman]

Correct me if I am wrong here but I would not put oil in the evap. Oil tends to migrate there as it is. The most important componant needing the oil is the compressor. However, I have said it before and I will say it again, too much oil is not a good thing. Please do not go nuts putting oil everywere. You will regret it!

[absolute-zero]

Correct me if I am wrong here but I would not put oil in the evap. Oil tends to migrate there as it is. The most important componant needing the oil is the compressor. However, I have said it before and I will say it again, too much oil is not a good thing. Please do not go nuts putting oil everywere. You will regret it!

I have stated this in prior post that only the olil amount that was removed should be put back in, not any more not any less.