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Joslaz
17-09-2012, 07:02 PM
Hi folks,

I am designing a cascade R134a/R744 system.
R744 side:DX system, working pressures -35C (evaporation) and -5C (condensation), The PS are 40barg and 27.5 barg for the high and low side respectively.
The compressor has a standstill allowable pressure of 30barg and moves a mass flow of 360 kg/h.
I want to install a condensing unit R404a for standstill to reduce pressure from -5C in liquid receiverf to say -15C (27.5 barg x 0.8 = 22barg @ -15C).
The liquid receiver, located in the engine room, has a volume of 45 liters and Armaflex insulation 50mm.
How to calculate the unit?

thanks a lot.

ATH
18-09-2012, 03:50 PM
Dear Joslaz, you have to calculate the surface of the vessel, know the k-value of your insulation and dt. difference between the engine room (ambient temp.) and you calculated vessel temperatur.

Q=A*k*dt

Joslaz
18-09-2012, 05:49 PM
Hi ATH,

Vertical liquid receiver: 323 x 698 mm, Asurf=0,87m2
Isolation: k=0,034 W/mk, so U=0,034/0,050=0,68 W/m2k
Max. machine room temperature = 35C (supposed)
Receiver temp, at standstill = -15C
Q = UAdT = 0,68 x 0,87 x (35-(-15)) = 29,58W???

What i do with this?

Thanks.

ATH
18-09-2012, 07:45 PM
Hi Joslaz, with this small capacities I do not have any ideas to give you, unless you have an other unit in the engine room where you can connect it upto.

ATH
19-09-2012, 08:10 AM
Hi Joslaz, install the vessel in a home freezer.

Joslaz
19-09-2012, 05:44 PM
Hi ATH,

This is not a typical solution. What about if receiver is quite bug and there is not a cold room?

dblissett
23-09-2012, 07:49 PM
Hi Guys,

I have two questions;

Does the R744 vessel isolate from the system during off cycle?

What is the total system capacity?

Also, when you are sizing a system it is prudent to build in some redundancy, perhaps allow for a 60% on cycle of your R404a system.

Thanks.