vango1

[vango1]

I am trying to build a -85C freezer. I have what I feel is a working knowledge of a cascade system, so I proceeded to buid one. I have experience with single stage systems, thus have built a nice freezer with a heat exchanger ( coil in a coil) in a well insulated box which I get to -41C. At this point I turn on the 2nd compressor which pumps the 508B into the condensor located in the -41C camber. The chamber drops to -28C the discharge pressure jumps to 350psi,. the suction runs around 11psi. Under these conditions I would expect the low side chamber to get very cold, but it barely gets below zero. What am I doing wrong????

[The Viking]

I think we need to know the subcooling and superheat of the low stage circuit.

[vango1]

I estimate my superheat to be 110C and the subcooling to be 70C.

[The MG Pony]

Could you please discribe in detail each stage and perhaps a half descant schematic.

Cascades are and are not as simple as they seem, I feel some thing is dierly wrong with yours with such temps that you are listing, some thing is certainly not correct about this.

[The Viking]

I estimate my superheat to be 110C and the subcooling to be 70C.

Vango,

The subcooling is the saturated temperature of your gas at the pressure in the liquid line less the actually measured temperature of the liquid line. (normally you would expect this to be in the region of 4C, less indicates an under charged system and more indicates an over charged system)

Likewise, superheat is the difference between saturated temp and real temp of the suction line.

[vango1]

Yes I agree with your definitions. I may have accidentally mixed units. So I estimated superheat to be 70C and subcooling to be 50C. Yes they are high ,but it is what it is. Today the heat exchanger is runing at -38C and the freezer chamber at -5C. Thats funny my heat exchanger is a better freezer than the freezer chamber.

[The Viking]

Not enough circulation of refrigerant.
Blockage in the capillary tube?