BPHE Expansion Valve connection - top or bottom?

[flyinkiwi]

I have a part of larger project for later in the year which has been forced upon us (myself and client) by deteriorating performance in the existing system.

Sooner than spend more money on the old system, he is happy to pay the extra for the larger size gear for his new site which happens later this year.

An accumulator will be fitted to the system.

The problem: Oil return within the heat exchanger with refrigerant upflow at part load.

All manufacturers recommendations state expansion valve to be connected at the bottom of the HX.

My problem is that it will be ultimately connected to a variable capacity system, which will be acheived with 3 or 4 steps (depending on evap temps, there are 2 different systems), and this raises a concern of oil return at part load when refrigerant is flowing upwards through the BPHE. Also, what is about to happen is we will connect it to an existing condensing unit of about 30% capacity, pending construction of new plant re-using this heat exchanger.

This will yield a velocity of approx 0.53m.s.-1 in the refrigerant side of the heat exchanger - less if at the 25% step. The large volume relative to capacity is a result of sizing for the correct pressure drop on the glycol side.

The question:
If I feed the evap at the top of the BPHE, will gravity have a negative effect on the distribution of the refrigerant across the width of the plate? Liquid return will be intercepted by an accumulator.

Thoughts and comments please.

[chemi-cool]

whitch refrigerant are we talking about?

[flyinkiwi]

As yet undecided HFC with POE. Most likely to be R407c, but could also be R404a, as I have 2 redundant condensing units to lend out until the new plant goes in at client's new site.

[chemi-cool]

One thing that help refrigerant to carry oil is Propane, I usually add some 2% by refrigerant weight and it does the job.
Another thing is to make sure suction lines diameter is correct, too large pipe will slow the flow.

oil separator is a must at your new system, head pressure control also, oil traps every 2 meters, increase super heat to keep the suction line with a small amount of liquid refrigerant as will help you to carry the oil up, add a small heater to the accumulator.
I believe this should do the trick.

[flyinkiwi]

Thanks Chemi.

I am only concerned about oil flow upward and out of the evaporator at part load when fed from the bottom - or refrigerant distribution if refrigerant inlet is at the top of the HX.

Pipes are no problem to get right, and it will be less than 2m total length to the compressor.

Head pressure control will be less important, as an EEV will be employed, and the valve is sized such that it can regulate correctly at the lowest possible head pressure that will be experienced.

[chemi-cool]

Good luck and keeps us informed.

[sandybapat]

you should use double risers in the suction line. This used to be common practice for R22 refrigerant. The single pipe sized for 50% (Partload) capacity and both together can be sized for 100% capacity. Refer ASHRAE refrigeration volume chapter on HCFC systems.

[flyinkiwi]

Thanks Sandy, but the problem is velocity and oil return WITHIN the heat exchanger, not pipes.

I'm hoping someone has some insight into the mechanics of refrigerant flow within the BPHE when it it is flowing from top to bottom as an evaporator, and any reasons why it can't be done.

I understand that feeding at the bottom as manufacturers recommend will give better coverage of the plates of liquid, but I am concerned about the impact of vertical flow within the heat exchanger on oil return at part load.

[al]

Given that the gaps between the plates are quite tight, even at part load would the velocity not still be sufficient to flush the HX? I've worked on many plate exchangers connected to screws, ramping from 10 to 100%, oil logging in the evap has never been an issue, although these tended to be flooded HX.

al

[HVACRsaurus]

With BPHE connected in the regular up flow direction, I'd expect any logged oil during low load to return should return when the load increases. If the plant has regular periods of high demand it's probably ok with bottom connection, but could be more of an issue with extended (days) of low load operation. If its a process load with periods of off cycle + periods on, the demand at start up should also send the oil back.

[hookster]

My thoughts would be that the lower connection point is correct and at partial load the high turbulence created inside the plate channels would provide the oil separation still even at lower gas velocity. obviously oil/refrigerant miscibility should be considered for selection.

Would gravity have a pronounced effect? I will leave that to you :D
http://dc.engconfintl.org/cgi/viewcontent.cgi?article=1036&context=heatexchangerfall2005

[flyinkiwi]

Thanks Al, Saurus, and Hookster.

When the new plant is built later this year, the PLC will be set up to operate 100% of compressor capacity periodically, to return any oil that may accumulate at part load.

The problem is what happens in the next few months, when it will never have more than 30% capacity connected.

If I connect it conventionally, I think I might choose the condensing unit with an oil sight-glass, so I can at least monitor the level. The other CU is larger, but doesn't have a sight glass...

See drawing below of what I'm anticipating if the BPHE is connected in reverse - only for the few months where 100% of compressor capacity is not available.
If this is what happens inside, and I'm not fully utilizing the heat exchanger, would I not be better to do this in the interim and suffer the loss of heat exchange area (not that I have compressor to match it - yet) and be certain of oil return?

[RANGER1]

With Alfa Laval BPHE with TX a distributor nozzle arrangemant is installed feeding each plate circuit.
I would imagine all plates are the same with PHE, so changing connections would not be possible.
The amount of refrigerant & oil in each circuit would be minute & think it would be self clearing.

[Magoo]

As per Ranger1, the PHE will come with an injection tube acting as a distributor with a linear slot sized for application, position of slot is important, definetly bottom feed. PHE's have low pressure drop so can flood easily. I prefer installing a evap pressure regulator as well, and a large suction accumulator

[flyinkiwi]

Haha. Yes, I'm waiting for McDowalls to FIND the distributor... they managed to lose one of the two boxes all the gear was coming in.
So far, I have accumulator and BPHE in hand - just hoping they don't take a week to turn around and say "we can't find the other box."

Edit: For those further afield, McDowalls is a freight company.

[sandybapat]

Dear flyinkiwi

Even if BPHE is having some oil accumulation and say only 30 to40% of the heat transfer area is available, that should be sufficient for your 30% load. They why to worry.

Another point, if your capacity control is on the fluid temperature at BPHE outlet probably the plant will balance to hiher evaporation temperature and the BPHE will almost operate to its full capacity because of changed LMTD. There might be better oil return. I mean the evaporation pressure should be allowed to adjust at higher pressure. If your capacity control is on suction pressure, probably you can set higher suction pressure. You can get your BPHE rated for this changed evaporation temperature.

I agree Al, oil return was never an issue in BPHE.

Sandybapat

[mad fridgie]

Bottom feed.
The amount of oil that BPHE can hold, is not so high, so running of oil for lubrication purposes is not reaaly goin to be a problem, if the BPHE just logs slightly, then you will loose a bit of transfer capability. But becuase you do not need it, do not worry about it.
But if you still have a bit of concern, you could at times allow your dischrage pressure rise, to increase the inlet vapour fraction to rise, driving the oil out of the bottom of the heat exchanger.