Late 1960's Reciprocating Compressors Discharge Temperatures

[Alan55]

Hi Folks,

We have Fedders 3 ton unit running nicely for over 50 years made in the late 60's. The unit is a stand alone unit encompassing the compressor, condenser, evaporator, etc.. The squirrel fans are controlled by one 4-5 amp motor that circulates air internally and cools the compressor/condenser thru 16" intake/exhaust ducts. The system is clean. It is a sealed system with no access valves. It still runs nicely (R22). The input/output air over the evap has 20 deg drop when indoor temp is 80. The condensor mid-coil is roughly 30 deg above outside temp, the evap mid-coil is 38 below indoor temp.

The problem with it was the low pressure switch on the suction line sometimes kicks in & cuts off compressor and when the temp outside goes up to the 80's, the discharge temp starts rising over 225. I asked you guys before and was told to have access valves put in by either recovering, having the valves brazed on, and then recharging and testing the system OR the other option was to use a "pinch-off" on a process stub to install access valves and then test. The tech that came said he could recover & install the valves, but was not familiar with the pinch-off. He was not too concerned with the discharge temp as the system may need charging.

The question is were these reciprocating compressors made in the late 1960's designed to handle higher discharge temps or 225 is the max? How much can the discharge temp possibly drop if it needs some charge?

The compressor has a process stub in good shape on the reciprocating compressor top side next to the suction line (discharge line is at the bottom). Can a pinch-off be used there for the suction side (metering is fixed capillary)? There are no process stubs on the suction line itself.

Thank you
Alan

[Bazooka Joe]

The compressors cover plate says internal and external overload instead of a just a internal one, like modern compressors seem to have in residential.

[pecmsg]

Get the suction S H in line and the problem should go away!

[Restaurant Mech]

Especially on something old I'd try and keep discharge under 180.
Cleaning the condenser may be all you need to do to achieve this.
The other thing is it may be a little low on charge. I'd be recovering and welding in proper access ports to get proper readings on sub cooling and superheat

[Alan55]

The compressors cover plate says internal and external overload instead of a just a internal one, like modern compressors seem to have in residential.

Hi Joe,

External being the LPC. Good to know.

Thanks
Alan

[Alan55]

Get the suction S H in line and the problem should go away!

Suction superheat was roughly 15 deg over target SH at the time. Hopefully just needs a charge & no issues with restriction on any of the 4 capillary tubes.

Thanks
Alan

[pecmsg]

Suction superheat was roughly 15 deg over target SH at the time. Hopefully just needs a charge & no issues with restriction on any of the 4 capillary tubes.

Thanks
Alan

so what was your target SH?

[pecmsg]

Hi Joe,

External being the LPC. Good to know.

Thanks
Alan

A LPC is not an external overload.

[Alan55]

Target SH 15.4...As I recall last time I checked with my brother on the elec schematic/circuit who is an EE found only the low and high pressure switches inline to cutoff the power outside the compressor...will ask again.

[rider77]

It is a sealed system with no access valves

Suction superheat was roughly 15 deg over target SH at the time.

How does one determine system superheat with no access valves? What is under the yellow caps?

[Bazooka Joe]

Hi Joe,

External being the LPC. Good to know.

Thanks
Alan

External overload is not the LPC, there is a internal overload and the external one is behind the cover plate, reason why there is a built up area between the cover plate and the actual compressor skin. Been a while, since I worked on one, but the external one(s) may be a for sensing over amprege. Modern compressors have 3 power wires feeding compressor, your has more wires going into that cover plate.

Easy enough to verify, shut power off and remove those three screws.

[Alan55]

How does one determine system superheat with no access valves? What is under the yellow caps?

Rider77

"The condensor mid-coil is roughly 30 deg above outside temp, the evap mid-coil is 38 below indoor temp".....I got those temps using thermocouples....thats why i say roughly. The actual testing SH at that moment of time was using the specific WB, suction, evap, outdoors temps. Those yellow plastic caps cover the production service valves that are permanently sealed with pins....its not magic :-)

[Alan55]

Thanks Joe...appreciate it.

[VTP99]

Put a piercing valve on the process tube and go from there.

[R123]

How does one determine system superheat with no access valves? What is under the yellow caps?

Measure the evaporator/cond temp on u tube in middle of circuit gives you saturation temp. Then you can figure SH/SC from there. Same way Trane RTAA Or Carrier 30GT chiller controllers measure it.

[pecmsg]

Under the pecker head is very Crisp wires. Be careful
SH at the pump still applies
20 - 40* at the pump
200 * discharge

[comfortdoc]

Clean the coils and blower wheel. Don't touch the sealed system.

[kdean1]

How far down the DIY road should we go in the AOP forum?

[VTP99]

It's been a month I think we reached the end of the road.