i work in a hospital that has various different ptac, mostly mcquay MEK009 MEK012.
I'm having problems with the units after i replace the compressors, tecumseh RK124et and RK222et.

I believe i narrowed the problems down to the expansion valve cap tube strainer area. I believe i was installing the wrong fixed metering devices. Now i have the parker type 1/2 ton.
Constant Pressure Expansion Valves

Inlet Conn.: 1/4" ODF; Mfg. #: ASB20; Outlet Conn.: 3/8" ODF; R12/R134a Capacity: 1/4 to 1; R22 Capacity: 1/2 to 1-1/2; R502/R404A Capacity: 1/4 to 1;

Order # :B13-672Mfg. # :ASB20Brand :Parker Hannifin CorporationCatalog Page # :1132Shipping Weight :0.50Country of Origin :UNITED STATES Johnstone Supply

before i was removing the cap tube off the strainer and adding new fixed expansion valve, no info was provided for valve but pressures were 300psig on high side and 20f on low side. so i would charge by weight only 28.5oz r22. the units ran ok for awhile, but eventually would come back overheating and compressor burnt out or hard start

i guess my question is when using these fixed devices how important and or whats the reason for the bypass cap tube or secondary metering device often flowing backwards in the refridgation loop.

times are tough so i need to keep it cheap, so cap tub would be ok, but i'm very inexperienced with cap tubing, loops and what not.

i pretty much do best i can with as little info as i have to fix these mcquays, but i need some help, maybe some tips and tricks.

the chassis have a seperate heat coil, which are mostly dirty and i clean them when i replace the evap blower motor. when those go bad i'm installing 1250rpm dual shaft motor because are vent grilles are very narrow for safety reasons.

thanks for any info