Strange TXV setup

[mad fridgie]

RLK, what we do not know is how difficult it is to insulate the suction line, only that he stated it was difficult, and we also do not know how important the water droplets are.
We do know that the inlet superheat needs to be dropped. If the suction line practically can not be insulated, and water droplets are an issue, then we have superheat the vapour, this can be achieved by placing the TXV bulb on the outlet of plate 3, adjust TXV as stated, there will be sufficient mass flow to freeze the last high temp eutectic plate, then the suction/liquid HX will increase superheat even further. We then need to reduce superheat at the compressor, it is common practice to install liquid injection (OK this normally directly into the compressor via a propriety device)
If he can insulate, then no need for liquid injection

[RLK]

Mad fridgie, What you do not understand is this system is not conventional balanced refrigeration system, it is a one ton compressor freezing maybe 70 pounds of ice in 4 small containers in less than one hour. In this type of one TXV four plate system heat is absorbed first in freezer plates than overflow saturated liquid vapor moves to the next warmer plate’s low pressure coil. Do to the mass refrigerant flow and poor thermo conductivity evaporator coils in plates there is always the possibility of vapor heat exchange in suction line. I only pointed out that the #4 orifice will at the beginning of one hour run increase moisture on return line. Whether suction line is insulated or not will have little effect on plates freezing process. Compressor temperature is expected to be very warm for several minutes as warm plate solution the first day is cooled down. If compressor remains hot till the end of compressor run SC in condenser is inadequate or system has too much refrigerant.

[mad fridgie]

Mad fridgie, What you do not understand is this system is not conventional balanced refrigeration system, it is a one ton compressor freezing maybe 70 pounds of ice in 4 small containers in less than one hour. In this type of one TXV four plate system heat is absorbed first in freezer plates than overflow saturated liquid vapor moves to the next warmer plate’s low pressure coil. Do to the mass refrigerant flow and poor thermo conductivity evaporator coils in plates there is always the possibility of vapor heat exchange in suction line. I only pointed out that the #4 orifice will at the beginning of one hour run increase moisture on return line. Whether suction line is insulated or not will have little effect on plates freezing process. Compressor temperature is expected to be very warm for several minutes as warm plate solution the first day is cooled down. If compressor remains hot till the end of compressor run SC in condenser is inadequate or system has too much refrigerant.

What you are trying to explain is a load profile, (same as ice builder, batch blast freezer) high at the start reducing at the end, this is fully understood, the system works well as far as cooling in the cabinets go, that has never been in question.
The suction line insulation and condersation is a practical issue, not related to the boxes, if he can not insulate and does not want condensation, then he needs to superheat the vapour, but he also wants his cabinets to work for he desired period. We have already proved vapour only out of the freezer plates will not freeze both chiller plates, some liquid is required, if liquid is passing through the first 3 plates then there is sufficient vapour at the right temperature to freeze the 4th plate, (superheating the vapour) the bulb on the outlet of plate 3 will control the valve.
If we look at his problems, if add more refrigerant it freezes his product (all agreed need some insulation on chiller plates)
Very high suction superheat, causing premature compressor failure, would be resolved just by adding refrigerant (and a few tweeks)
Then he has condensation problems, but has difficulty insulating the suction line, "catch 22" solution as stated install liquid injection at the compressor (i said accumulator inlet, to allow for a small level of play in sizing) if I suggested at the comp inlet and cap selection was incorrect he could end flooding the compressor and were are back to the start again.

[desA]

We have very high superheat!

Add a small TXV or capillary and inject into Accumulator inlet which is adjent to the compressor, the will then reduce SH to an acceptable level at the compressor inlet.

How low would you allow the SH to go, in this case?

Where would you place the existing txv bulb?

Could the evap/accumulator be allowed to run wet, with TXV bulb placed at outlet of accumulator?

[mad fridgie]

How low would you allow the SH to go, in this case?

Where would you place the existing txv bulb?

Could the evap/accumulator be allowed to run wet, with TXV bulb placed at outlet of accumulator?

This was only needed if insulating the suction line was not practical. (answers based upon this assumbtion)
1; Superheat range at the compressor 5-15C
2; Bulb position of existing TXV outlet of plate 3
3; With a TXV fitted to acc, bulb would be at acc outlet, not really wet (refrigeration wise) I would partially insulate the acc. (I hear arguments coming, the acc is really a knock out pot, even with a bit of insulation liquid will boil off during long off periods)

[desA]

This was only needed if insulating the suction line was not practical. (answers based upon this assumbtion)
1; Superheat range at the compressor 5-15C
2; Bulb position of existing TXV outlet of plate 3
3; With a TXV fitted to acc, bulb would be at acc outlet, not really wet (refrigeration wise) I would partially insulate the acc. (I hear arguments coming, the acc is really a knock out pot, even with a bit of insulation liquid will boil off during long off periods)

Many thanks, MF.

Would there ever be a situation where SH, as measured at accumulator outlet (wet acc), where SH would hover ~ 0-1K?

[mad fridgie]

Many thanks, MF.

Would there ever be a situation where SH, as measured at accumulator outlet (wet acc), where SH would hover ~ 0-1K?

Very common, "flooded systems" by design, practically many refrig systems, due to a number of reasons, incorrect valve selection, low loads, fixed orifice types.

[desA]

Very common, "flooded systems" by design, practically many refrig systems, due to a number of reasons, incorrect valve selection, low loads, fixed orifice types.

Thanks again.

Would this be a bad thing, for extended periods, or just part of the way things are?

Should this be taken care of? If so, how? (In other words, is it difficult to 'rip' the SH off the 0-1K position?)

[mad fridgie]

Thanks again.

Would this be a bad thing, for extended periods, or just part of the way things are?

Should this be taken care of? If so, how? (In other words, is it difficult to 'rip' the SH off the 0-1K position?)

This really is down to 2 things, absorbing refrigerant into the oil, loosing lubricating properties and the chance of forming liquid in the compression chamber.
It is "deemed" as a bad thing, if refrigerant is direct in to compression chamber, less effect on the oil and if the compressor is in a high ambient (above SST) then less chance of liquid condensing in the compression chamber.
getting close to my limits of knowledge on specifics, at this point on rely on component manufactures recomendations. it maybe worth starting a new thread, I am sure there are those out there who know more about the specifics

[mad fridgie]

Missed the how bit,
A simple pressure drop or what i was taught a cooling off coil, which actually meant a warming coil,
Suction/liquid heat exchanger

[desA]

Thanks again, MF. Was just wanting to cover the bases on dry/wet accumulators & consequent high/low SH - with possible effects on compressor.

I'd expect that the determining SH would really have to be at final value taken at compressor suction & a judgement call made.

I see Sporlan have an injection valve for this purpose - TREV Y-1037.

[desA]

Missed the how bit,
A simple pressure drop or what i was taught a cooling off coil, which actually meant a warming coil,
Suction/liquid heat exchanger

Thanks for that.

[Chef]

Mad Fridgie - a first look at the cap tube idea shows we might need between 1/8 of system flow for a 10K drop in SH and 1/4 of the system flow to get 20K drop in SH, both quite high relatively and will drop performance of quite a bit.

The suction line is built inside a wood former and water dripping of it would eventually cause damage - not good and then it passes by some sensitive equipment - although this could be shielded its again not ideal.

So I feel it has to be best to just cut it off and run a new suction line where it can be insulated. Solves several problems in one go. I am pretty sure there is a way to run it if we change some steel supports to get access.

Shame as the challenge of the injection system was of interest.

RLK - I fully get the plate freezing process and ice generation from plate 1 thro plate 4 as the system cools down but putting the bulb on the outlet to the freezer is out as the SH will not be enough to cool the last 2 plates.

Anyway - small changes are not possible as the system is now laying on the floor in pieces - so it has to redesigned and built in one go and all the correct changes made in one giant leap.

Chef

[desA]

A thought, Chef.

Sketch your proposed new circuit layout. Take pics of all the parts. Paste them up on the board. You'll be able to settle your design fairly quickly I'd think.

[mad fridgie]

Good that you are replacing the suction and insulating, does save a number of hassles.
Just a point for your own interest, liquid injection would have very little effect you net refrigeration, The refrigerant would be more dense entering the compressor, thus more mass flow.
I would leave leave bulb on the outlet of the fourth plate (then play which side of the hx, keeping SH at reasonable level)
You have to keep her indoors or should i say onboard happy!

[RLK]

Chef, it is always difficult to convince refrigeration service techs that on series holdover plates there are other ways to control superheat and flow. There is also no comparison between bulk ice production load profiling and freezing a -20 degree eutectic solution with 134a refrigerant in a system. All this is a mute point since system is apart and can now be redesigned. Over the years I have designed more than 100 engine drive boat refrigeration systems. There are over 60 different in service boat holding plate designs in my fourth DIY boat refrigeration book. Although I have built many systems with three plates in series, I recommend separate TXVs for each box to reduce daily engine run time. The valve orifices would both be #01 on your dual valve system. After the suction line T connecting all plates return lines back together I would use at least 7 ft of 5/8 tubing to compressor’s vibration flex tube or install a suction accumulator in this line.

[Peter_1]

I've not written DIY books and have not installed hundreds of these systems - well, at at least 30 and we made most of the plates ourselves - but why make it all that difficult.

Move TEV to the hottest place, eventually in the compressor compartment, leave the plates in line and select the orifice slightly a little too large and install a suction accumulator. Did it this way tens of times without any problem. This is not a high tech install, isn't it? And cover the chill plates while charging them. Once charged and compressor disconnected, they will rise within 15 minutes to their eutectic point.

If you're concerned about high SH, you still could install a TEV with SV injecting its liquid at the inlet of the suction accumulator to control SH. Posted in other posts on RE pictures of this (capacity controlled compressor)

[Peter_1]

Forgot, do a pump down or install an SV in the liquid in line with the compressor, otherwise the liquid will migrate to the cold plates. (Perhaps this was already mentioned in earlier posts but I will not read all 200 posts)

[mad fridgie]

I agree with Peter, this is not a complex system, if we had been on the boat, all would of been sorted over a beer or two, it is somewhat easier to resolve problems, when you can actually see. For example replacing the suction line, I do hope those who have been watching on the sidelines have found it informative.

[Peter_1]

These are very learn-fully posts (is this the correct expression learn-fully?) for everybody.

[Gary]

These are very learn-fully posts (is this the correct expression learn-fully?) for everybody.

I believe the word you are looking for is "informative".

[Gary]

Forgot, do a pump down or install an SV in the liquid in line with the compressor, otherwise the liquid will migrate to the cold plates. (Perhaps this was already mentioned in earlier posts but I will not read all 200 posts)

the good news is that the charge will migrate to the freezer plates (coldest plates). Since it then has to go through the warmer fridge plates, there should be no problem with floodback on startup. So, a pumpdown is not needed.

[IMSTIEN]

Gary I like your design with two txv's and the Epr valve.I have done some modifications of this sort in the past It works like a charm.

[sterl]

This thing would be a monster to design: Need to know the volume each palte, the volume each well of glycol and on and on...

Simple to find out what changes with the TXV diaphragm near liquid temp: Heat the power head, temporarily. The actual temperature not a detail concern as long as it is above the anticipated max bulb temperature during operation.

Almost certainly the system running critical charge, and TXV is at near-fixed postion once suction pressure has dropped during the restart; and once the coldest section has frozen out: the area of refrigerant evaporation moves on to the next-warmest and so on. Also means the coldest, now frozen plate partially full of liquid refrig; thus circuit will act short charged and receiver volume would be of benefit.

But other than establishing the point: Why bother? I assume the compressor that fails every year orr so is a Wobble plate? Acting at 2-4 Psi Suction? Yes, could be an issue....