Ok, I guess this turns into "Flooded Screw 101", but the questions you are asking come up often enough that it may be worthwhile.
You are right, anti-surge protection is a concept limited to dynamic compressors and isn't used in positive displacement machines (like flooded screws). People put recycle valves on PD compressors to limit hp by dumping "extra" pressure back to the suction. As I learned in Ariel school:
The best form of capacity control is to never compress the gas in the first place (vary rpm to match required load). Second best is to reduce volume early (with a slide valve or turn valve to dump it from near the bottom of the screw travel; the recip corollary is adding clearance). Third best is to restrict suction pressure (which can waste a huge amount of available energy). The worst form of capacity control is a discharge recycle because the pressure you are dumping has had a bunch of work done on it.
If your recycle valve is open very often, then you need to adjust your VI to lower the discharge pressure.
On suction controllers for capacity control, I've seen them double the driver hp requirement by taking a 90 psig stream down to 5 psig and then the compressor had to boost it back to 85 psig to get into the line--a bypass would have made a lot of sense
The thermal control valve is a 3-way valve with a "pill" in it that senses oil temperature into the process (oil pump suction if there is an oil pump) and increases or decreases the amount of cooler bypass that is allowed. In refrig service, I see a lot of 140F pills that will try to maintain the temperature into the process at 140F. For refrig service, this is a reasonable approach since the heat is not required to cook water off.
There are a lot of assumptions in the choice of pills (they are not adjustable, you can adjust the process by changing pills) since the pill is looking at the cold stream and the packager can assume a compression ratio (and therefore a heat of compression and temperature rise). This is fine in a very controlled environment with dry gas. In blood-guts-and-feathers well stream use, the suction and discharge pressure change continuously and the amount of water vapor in the gas is very high, so as the oil absorbs the water vapor and then never gets hot enough to cook it back out, the oil properties rapidly degrade. In wet gas service it is the single biggest cause of failure and oil loss because the bypass scheme rarely results in the desired 200-210F discharge temperature. I really hate 3-way valves in wet-gas service, but they are fine in a controlled environment like propane refrigeration
In refrig service, both the suction pressure and the discharge pressure tend to operate in fairly narrow bands. Packagers take advantage of this by using dP across the skid to drive oil flow and don't install an oil pump. That dP is typically quite adequate to allow the oil to perform all of its required functions (lubrication, seal the rotors, and carry away heat of compression), and with a reasonably constant dP they can control oil flow-rate with orifices instead of more expensive control valves. The dP sensor is probably tied to a Low-dP Kill.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The main difference beteen humans and apes is that we have cooler tools
