R-23 to R-508B

[Rob S]

I'm looking for advice on changing over some chambers from 23 to 508B. I was told by a person to just use 80% of the original charge. After completely rebuilding one chamber out of 25 this person worked on for years. I have no faith in this persons thought proccess.

The Thermotron units are listed as 8lb charges with a 104psig static @ 21C I'm assuming. Thermotron wasn't any help to me. Becuase I'm not a slave to their will I guess.

The Especs are mostly 5lb charges but there isn't a static rating in the manual or on the nameplate.

It's been a while since I had to calc out this stuff. R-23 @ 120psia and 70F (21C) has a V = 0.6320 R-508B has a V = 0.4622
So (0.6320/1 = 5.056/8) (0.4622/1 = 5.056/x) x=10.938

I would think there would be a need to change some compontents on the low stage as well. Main TXV, low cooling captube, and expantion tank.

[phil68]

It's been 17 years since I worked on Thermotron chambers as I left their employ in '89, but from memory the static charge was 125 psig @ 70 deg. F on R13 back then. Not familiar with R508B -are the evaporating temps. & pressures very similar to R23? If they are then the TEV could propably do with just a tweak & that's it. The small liquid line cap tube would be OK left untouched as it only comes into play when nearing setpoint & to control @ setpoint. The expansion tanks also. I do remember that when converting from R13 to R23 the charge was 80% smaller, but I'm not familiar with R508B.
I can give you Thermotron UK's phone no. if you like, they may be more helpful. Their offices are 3 miles from where I live

[Rob S]

As similar as say.. R-12 is to R-134A
At -100F(-73C) R-23 = 8.5psig R-508B = 17psig

[Andy]

Hi Rob S
if I was in your situation I would add a 50% charge of the new refrigerant by weight, compared to the old charge. Run the plant to set point and see how it performs. At 50% the superheat leaving the evaporator will be high, alsdo the air offs. Charge the plant until you get an aceptable air off, compared to a not so high suction pressure as to cause the compressor(s) to exceed the original design.
Mind if you add gas to check the pulldown cycle again, so that the pressures are not too high on start up.

Good Luck

Kind Regards. Andy.