System Design Pressures

[herefishy]

On refrigerating equipment, you often see high-side and low side design pressures indicated. How is this information determined?

[chemi-cool]

I belive its the result of testing by the manufacturer.

this should be the safty zone. it will probably work over the pressure indicated.

i'm sure you have seen units work in sommer over the pressure indicated on the parts and nothing happend.

when high pressure control fails, and it does, the pressure jumps to the sky before the overload trips.

I guess they're trying to warn us.

chemi

[herefishy]

Well, I'm working with a manufacturer, who is trying to determine how to establish this information for submitting a machine to UL and NSF.

I'll do a little more research.

[chemi-cool]

what is UL and NSF?
maybe I can give you some directions.

chemi

[herefishy]

UL = Underwriter's Laboratories www.ul.com

NSF = NSF International (National Sanitation Foundation) www.nsf.org

[chemi-cool]

so, your friend should get the design plans to the UL and NSF.
and after they will check al the details I'm sure they will tell him exactly what he needs to get and do to get his machine on the road.

the rest of the world has ISO. I think there is no ISO in the USA.

chemi

[herefishy]

Design Pressure : Highest or most severe pressure expected during operation. Sometimes used as the calculated operating pressure plus an allowance for safety.

The system is a cold-wall evaporator with forced air condenser. 3.5 oz R-134A.

Is the design pressure a guess?

[Andy]

Hi Mark
from memory you decide on your design or operating pressure, you then look at the highest condensing, we usually go for 45 deg C for air cooled and 35 to 40 deg for evaporative condensers (and evaporation where applicable) this will be 0.9 times your maximum allowable pressure and the settings of your H.P safety. Your releif valve is set at AP and you strength test to 1 to 1.3 times your AP.
Hope this points you in the correct direction.
Kind Regards. Andy.

[Peter_1]

This is the new law in Europe
http://ped.eurodyn.com/
Perhaps you can learn something from them. Haven't looked yet around on their site.

Peter

[herefishy]

Thank you, Andy, Chemi,... and - Peter_1, that is a very extensive site, you link to. Like all the other bureaucratic (sp?) enviromental/safety sites, it'll take a month to see if I can find what I'm looking for (then likely I will have to pay $250.00 to see the text ... LOL )

I going to test Andy's suggestion. Here in the US, we are used to seeing this information (design pressure) on the data plates on the equipment that we service that is factory assembled or feild assembled as a factory unit.

Cheers

[herefishy]

Okay.... I ran some simple math (which is about all that I'm capable of), and decided that I would not expect the lowside to be exposed to more than the saturation pressure of R-134A, while the machine is idle, next to the pool in central Texas with the Sun beating down on a black shroud (130F/55C), applied Andy's safety factor of .9, and came up with a design low-side pressure of 180psig.

I looked at the critical pressure of R-134A, and determined that that was not really "expected". I got a brainstorm, and went to the Parker copper spun drier dept., to get their "design pressure" of their product, 500 psig. Employing "Andy's" safety factor, I came up with 450psig for the design pressure of the high side.

Sounded good to me, so that is what I recommeded.

Cheers, and many thanks.

[frank]

Herefishy

This was copied from the UL link you posted - does it answer your question?

Note 1: For other refrigerants, the minimum design pressure shall be no less than the saturation vapor pressure of the refrigerant at the following temperatures:

26.5°C (80°F) for low-sides.
51.7°C (125°F) for high-sides.
40.6°C (105°F) for water or evaporatively cooled high-sides.

Note 2: The design pressure need not be greater than the critical pressure of the refrigerant unless the critical temperature is exceeded under operating, standby or shipping conditions (DOT shipping conditions are 130°F). In which case the design pressure will be determined by the pressure at the operating conditions, standby conditions or 130°F whichever is highest. (Reference ASHRAE 15, Clause 9.2.2.

Note 3: For other refrigerants not listed, see the related standards Safety Code for Mechanical Refrigeration, ASHRAE 15, and the Mechanical Refrigeration Code, CAN/CSA-B52.