Constant pressure or Hot Gas

What is the difference between a hot gas bypass valve, back pressure regulator and a constant pressure expansion valve?
They all seem to do the same thing by opening on drop of outlet pressure

Hi Drew
HGBPV simply bypasses the condenser into evap for defrost
CPEV acts like a MOP TEV (maximum operating pressure) limmiting initial startup load, used on some chillers to prevent loading up to quickly causing shortcycling
And the BPRV limits suction pressure at comp getting to high after a defrost
So in short the CPEV closes on rise in pressure. I think thats right (correct me if Im wrong)

Thanks for the reply.
Here is a link to a bit of info Ive found with regard to Pressure regulators and Back pressure regulators.:

http://www.plastomatic.com/bprvspr.html

Sorry Drew I might have got it slightly wrong by the look of web site. Looks like Back Pressure RV controls upstream and Pressure RV controls downstream. Then we have normally open and normally closed valves. More comments on this appreciated.
Thanks

So many brands, all with different desciptions of their valves. Does anyone know where I can get a brief overview of the different style of valves.

A good source of information is the old Sporlan site here...

http://sporlan.jandrewschoen.com/literature.shtm

thanks Brian_UK, just found out where Nike123 gets his field chart from ;-))

Ice