Probably should be talking PSID (D for Differential), vs PSIG (G for gauge). I think that is reason for the confusion.
Last edited by Nuclrchiller; 11-08-2012 at 01:17 PM.
Reason: clarified with "probably".
Shockwave, remember it is a "differential" switch so it isn't looking for just 60 psi. It is looking for 60 psi "difference" between suction and discharge side of the pump.
I'm going to bet the "20 psi" isn't "cut out". Instead it should be the switch differential.....meaning once you reach a diifference of 60 psi between suction and discharge the switch changes state (say it closes).....once the difference drops by 20 psi the switch changes state again (in this case it opens)....as in 40 psi difference between suction and discharge on the pump.
Without more info I can't guess what its purpose is in your BAS. I doubt it is used to stage a chiller on/off as each chiller should have its own differential pressure switch. Likely it is just used to activate an alarm to let you know the pump isn't performing at desired level (for any number of reasons).
We use dp switches, or other methods, to prove the pump is working in our building control systems. In a lead/lag setup if the pump is not proven within a reasonable time frame then the next pump is called for, and it needs to prove its operation. It also generates an alarm on the "failed" pump. That alarm is sent to 'He who needs to know'.
DP switches aren't the only way to prove flow through a chiller condenser or evaporator. There are also mechanical paddle switches and thermal dispersion sensors like those made by IFM Electronic. All 3 of them are very common and are still widely used. They are also selected for different applications. I prefer IFM sensors or mechanical paddle switches in places where the static pressure in the system is too high to use a standard DP. DP switches all have a "proof pressure", which is the highest differential pressure either of the bellows can be subjected to without damaging the switch. Switches with a high proof pressure are usually much more expensive than the standard ones. DP switches also suffer from setpoint drift pretty badly after a few hundred cycles. I typically pull them and re-set them with nitrogen once a year. As was mentioned previously, the piping that goes to the DP is also a maintenance item. It frequently needs to be cleaned out.
Some manufacturers will even tell you that their chillers are smart enough to not need a flow proving device at all.
"There is no greater inequality than the equal treatment of unequals."
you could always print it?
Originally Posted by Scotty58
I WILL SELL WORK,GENERATE BUSINESS, GO GET NEW CUSTOMERS!
YOU SHUT THE HELL UP AND QUIT RUNNING YOUR MOUTH!
The most important thing you need to know about chillers is, that water freezes at 32°F and static water freezes far quicker then anything else. Therefore never charge a water chiller with liquid refrigerant at saturation temperature below that magic point, never do that with static water on cooler and condenser and never bypass the flow protection device (of course the HP switch as well).
"Quality exists, when the price is long forgotten."
remember that dp switches on pumps prove that the pump is RUNNING. they do not prove that the pump is FLOWING. i have been to many jobs where the bas techs use dp switches to prove the pump is flowing (which is incorrect) and then start the chiller. i mostly don't like them on anything (pumps or chillers) because the piping to the switch does not have any flow in them and over time, crud fills up the piping making the switch fail.
a fun example with dp switches on pumps: close 1 isolation valve to the chiller (with the chiller off) then manually start the chilled water pump...the switch will make...do this in front of the bas tech and ask them to explain why this is happening. 95% of the bas techs will say that the switch has failed and needs replaced!
ifm effectors have been my favorite for about 5 years now....they are great if selected properly and installed properly.
"If you pull one more stunt like you just pulled with Tommy, you won't have to get on a plane because I will personally kick your ass from here to Korea!" - Best of the Best
They are old tecnology. We typically use VFD's and pressure transducers, as well as flow meters, but his question was about why he had a DP switch going to his BAS. Question answered?