Design pressure for cascade systems according to ASHRAE 15

[adrianKwia]

Hello all,

I'm designing the cascade system for the US market and I have some concerns regarding design pressure according to ASRHAE 15 standard. It says that:
"9.2.6.2
a) The high side design pressure shall be not less than 120% of the maximum pressure developed by a pressure imposing element.
b) The low-side design pressure shall be not less than 120% of the pressure at maximum operating conditions, corresponding to the warmest location in the circuit."

How to determine "the maximum pressure developed by a pressure imposing element"? Is that a pressure which I set on pressure limiting device or is it design pressure of compressor itself?

The part b) does not make any sens to me at all. I'm not a native English speaker maybe that's the problem. Could someone explain it in simpler words?

Thank you guys for your help in advance!
Best Regards,
Adrian

[MrFreez717]

Adrian, I agree that the ASHRAE code for this is very poorly written and quite ambiguous.

I have done a couple of these systems here in the US. For the high side, we used the design operating pressure (25F or about 441 psig) x 1.2 and rounded up to 600 psig. If you are going to try and develop hot gas for defrost, your design will probably be more like 40F (553 psig) and you might round up to something like 50 bar (725 psig), which is still within the useable range of most common valves like Hansen or Danfoss.

The warmest location on the low side of the circuit was usually -20F or below. We used 300 psi for this, if memory serves. But you might want to consider something higher if there is going to be any off time for this system. We had to put standby condensing units in these systems to keep CO2 LPR pressures below the relief setpoint during idle times.

I can't swear that what we did was exactly in line with the intent of ASHRAE, but in my opinion, that's ASHRAE's fault for not writing a better code. The inspecting authority (local fire department) had no idea what any of this meant, so they didn't argue about it.

Good luck with your system,
MrFreez717

[adrianKwia]

Thank you MrFreez, for your answer!

If I understand correctly you did system witch CO2 on the second stage. My design is quite different, I'm going to use R23+R14 blend (R14 to go below -80 celsius without using 3rd stage). Currently I expect operating high pressure to be up to 21 barg (305 psig). So giving 20% buffer my Desing pressure should not be lower than 25.2 barg (366 psig). The system is also designed as a "limited charge system", I have a buffer tank to store evaporated refrigerant while the system is turned off, so there is no need for extra chillers to keep it liquefied.

I would like to not exceed 400psig design pressure. Most UL and ASME certified components which are available in Europe have 400pisg rating. Importing components from US takes a lot of time and usually cost a lot.

Does my system design sounds good to you and I can stay with 400psig design pressure?

Thanks,
Adrian