Difference between open flash and closed loop economisers

[Rex001]

Hi All
Can someone explain the difference between open flash and closed loop economisers and where should you use either or. And how can you tell if it’s open flash or closed loop.

Appreciate the help
Thanks

[GrantD]

Hi Rex,

There are two basic types of economizers for screw compressors:



Open Flash type
Closed Flash type



The closed flash type is relatively simple. High pressure liquid from the receiver is fed into a vessel through an expansion device. The liquid flashes off from receiver pressure down to the pressure in the vessel (set up by the compressor). The flash gas flows out of the vessel and back to the screw compressor side port. The cold liquid is at an intermediate pressure (between suction and discharge pressure). The liquid is saturated at this intermediate pressure. It is not subcooled, but rather a cold saturated liquid. The low stage vessel liquid feed is taken from this vessel. Larger control valves required for both the low stage vessel and economiser vessel due to the lower delta P across the valves and the fact that the economiser vessel must see the full mass flow of the low stage load + the economiser load (flash gas).

The closed flash type uses a coil which is submerged below the liquid level in the economiser vessel. This coil acts as a heat exchanger. High pressure liquid from the receiver flows through the coil. The other side of the coil is simply an evaporator (the coil in the liquid). This type of economizer only subcools liquid from one temperature to another lower temperature at approximately the same pressure. The gas that is created by cooling the liquid (in the cooling section of the heat exchanger) is diverted back to the side port of the compressor.

With the open flash economizer, the liquid is at saturation temperature so the liquid can still flash off due to heat gain or pressure drop in the liquid line.

With the closed flash economizer, the liquid is subcooled and will remain so until the liquid has warmed back up to the saturation temperature of the receiver pressure, or if there is a lot of pressure loss in the piping. The liquid in the coil is subcooled to within 10K of the saturated temperature of the economiser vessel pressure.

My personal favourite is the one flash type. Slightly more efficient (improved compressor C.O.P) than the closed flash type. I have had the closed flash type have a leaking sub cooling coil in the vessel (recently I might add).

There may be a need for a closed flash type say if the low stage vessel is a distance or possibly installed substantially higher than the economiser vessel.

There are also other ways of sub cooling the low stage liquid feed say through a PHE.

I hope the helps! I will try dig up some drawings and show you the difference. When insulated there will be some tell tale vessel nozzles, etc.

[RANGER1]

Check out previous posts as well as Grants response.

https://www.refrigeration-engineer.com/forums/archive/index.php/t-12688.html

[Rex001]

Brilliant Grant, can’t thank you enough thanks for your response. Nice explanation on the both of them. Cheers

[Rex001]

Thanks for the link ranger. Can’t have too much information.

[GrantD]

Hi Rex,

No problem. I noticed a typo. I hope you picked it up. The first one I explained is the open flash type. The closed flash type is the one with the submerged coil below the economiser vessel liquid level.

Cheers.

[RANGER1]

Hi Rex,

No problem. I noticed a typo. I hope you picked it up. The first one I explained is the open flash type. The closed flash type is the one with the submerged coil below the economiser vessel liquid level.

Cheers.

Grant,
Mainly use closed economiser, as difficult to feed liquid under low pressure any distance or height.
Have only seen years ago on single stage economiser, accumulator with liquid pumps very close.
Also these days better modulation valves on economiser & vessel makeup to keep things smooth.
Not on/off with solenoids & float switch.