Brazed Plate Evaporator - Need Phase Separator?

[SES Utah]

Take a look at the drawing. If I am using a brazed plate heat exchanger as the evaporator, does it make sense to use a phase separator prior to the heat exchanger so that I do not have two phase flow entering it?

Phase Separator.png

It seems like the phase separator is just an accumulator in a flooded evaporator... Is that right?

The only downside I can see is that the gas which is bypassing the heat exchanger does not get any super heat... But If I have a heat exchanger dedicated to superheating the gas, then this is not a problem.

[Peter_1]

This is one to discuss for DesA, Chef, Mad Fridgie and Chillerman

[desA]

Hi 'SES Utah'.

Would you first like to plot the system on a log(p)-h diagram? This will give us something concrete - in thermodynamic terms - for discussion purposes.

[chillerman2006]

Hi SES

Often getting things upside down lately, so bare with me if I have again,

A 'phase seperator' (or Economiser) on the liquid line, has the benefit of lowering flash gas content of liquid + additional subcooling, feed to the TXV, Normaly this will have tubework running direct into the side of a screw compressor (for instance)

Mad Fridgie has come up with an ingenious idea, that the flash gas can do additional work !

Where it is fed into (under development) a type of mixing vessel on the suction line, the flash gas is used as the motive force to increase flow of the refrigerant passing through the evaporator

This would then give you increased latent cooling efficiency, higher velocity (more efficiency) and increased efficiency from the compressor by bringing the working pressures closer together

There is still some R&D to be completed, but it has been proven to work !

'DesA' & 'Chef' are the Brains behind proving the calculations & 'Mad' the man that can (Idea's + R&D & could actually put the concept into pratice for your company), 'Moi' was just the office girl with the legs !

R's Chillerman

[mad fridgie]

This may be simpler than you think, it all depends upon your design selection of the heat ex-changer.
Without knowing the operating conditions, it hard to say if you need to add extra super heat.
You could look at an over feed system (no evap superheat), and use a Philips ejector to draw the extra liquid form the separation vessel

[chillerman2006]

Hi SES

was in a rush this morning (nothing new) & upon a closer look at your drawing



If You have 'no' superheat in the vapour, returning to compressor from a low side 'phase seperator' is there not the risk that the vapour could be highly saturated & holding too much liquid in the mix ???

R's chillerman

[SES Utah]

This is for a 3 stage cascade. The mid and lower stage will have superheaters (recuperators) that superheat the return gas by cooling the incoming high pressure refrigerant. The recuperators will add over 30 degrees of superheat.

I'll get a PH Diagram when I finish updating my model, desA.

[desA]

Can generate the process pretty much by hand. Helps to understand what is fundamentally going on.

[mad fridgie]

Ah all comes clear! (well as mud), same answer it comes down to how you select your heat exchanger.
The flash gas, in the heat exchanger either can be viewed in to 3 ways, potential increased pressure drop through the HX, or increasing the internal turbulence within the HX (altering the heat transfer co-efficients) or add some additional net cooling load "useful superheat", If the the above are not issues I would not bother with the vessel, just pass it through the heat exchanger.

[SES Utah]

Thanks fridgie!

I had a quick discussion with the evaporator manufacturer and he said that if we are careful with our piping that a phase separator was not necessary.

I think I will remove them from the design and see how we do.

[mad fridgie]

we are friend here, you can call me "Mad"

You main consideration is the pressure drop and how this effects the boiling temperatures across the plate heat exchanger. (if high this can change quite dramatically)

[Villhelm]

BXV separator.jpg

Hi Gentlemens
If you use standard plate hx small angle, and dimension for dry exp valv td 7K and dont use choke-ditibutor there is very small pressure-drop with nominal load . My experience is about 0,5K so if you use parallell flow heatexchanger and td in about 2,8-3,2K the heat starts up boiling refrigerant quite easy. Phase separator is only gravity down to bottom of accumulator.
Best wiches and good luck
Villhelm