Strange TXV setup

[Gary]

Just run this one by me - 2 TXV's?
an EPR why.

Are you sure this will achieve the same thing as we have now with the differences in temp between the plates.

I must admit the charge critical solution is not ideal but it does seem a really good solution and I just cant see how 2 valves is going to replicate it.

All the plates will still get down to -20C

Chef

I am suggesting two parallel circuits.

The first circuit: liquid line>TXV1>Plate1>Plate2>suction line.

The second circuit: liquid line>TXV2>Plate3>Plate4>EPR>suction line.

The EPR maintains higher temp in Plates 3 and 4.

[RLK]

I am suggesting two parallel circuits.

The first circuit: liquid line>TXV1>Plate1>Plate2>suction line.

The second circuit: liquid line>TXV2>Plate3>Plate4>EPR>suction line.

The EPR maintains higher temp in Plates 3 and 4.

Gary, is correct standard practice is a split system with engine driven eutectic holding plate.

In addition to dual TXV’s a solenoid is a good feature in liquid line to refrigerator TXV controlled by thermostat located in refrigerator.

On holding plate system, suction pressure regulated by an adjustable low pressure switch set to stop compressor when freezer plates are frozen solid is used to control system.

These systems are generally water cooled so a high pressure switch set slightly above max operating temperature.

I assume the compressor is rated from 6000 to 12000 Btu at 2000 rpm when used in a High Back Pressure application. I also assume you are using a swash plate compressor like one of the Sanden models. In a properly designed refrigeration system these compressors have operated without failure for more than twenty years. Repeated compressor failures on eutectic plate systems are caused by:

Incorrect oil level, The oil cavities of these compressor are designed to hold less than 7 ounces oil, in these mutable plate systems additional oil is needed as oil and refrigerant velocities at low plate freezing temperatures is not adequate. Adding to oil return problem there is often many oil trap areas in eutectic plates.

Normal cycling of a eutectic plate system compressor runs one or two hours per day and off twenty two hours. During off cycle refrigerant and oil migrates into very cold plates remaining there until the next startup. Since there are no restrictions in suction side after TXV liquid in plates accelerates rapidly back to compressor. To reduce the possibility of compressor liquid flooding these systems need a suction line accumulator and TXV orifices that are sized to max plate Btu acceptance rather than compressor max capacities.

The engine drive refrigerator compressors that I know of are operated at an rpm that matches maximum Btu that plates can absorb. On a system where the aggregate of all plates eutectic solution is 15 gallons, normal compressor operating rpm is 900 to 1200 with a max short duty cycle or 1600 rpm.

[Chef]

RLK - many thanks for your info.

The system has a low pressure switch but is normally always manually controlled, even so it does drop out if the suction goes past 0PSI. The high pressure switch is set to allow initial pulldown from warm and is about 225PSI.

It is water cooled and the condenser seems fine as the liquid out is just a few degrees above water temp.

The unit is a TM13 which is similar to a Sanden SD5. It is rated at 14,000BTU at 1800RPM for HBP applications and we calculate we are getting about 11,000 to 12,000 BTU at our conditions. The eutectic capacity is 8.6 USgal in all 4 plates

The max BTU acceptance of the plates is unknown at the moment as the coil arrangement inside the plate is unknown and some are sealed so we can never open it to find out. The only way would be to measure the pipes internal volume once the system is apart but it is an approximate method only.

The compressor failure by oil hold up is possible but whenever we have checked it the oil quantity seems OK for the unit - normally we test about 2 or 3 oz in the compressor which is pretty close to the specs for an SD5. However we always test the oil immediately after a cycle so the unit should have the correct oil returned to it and maybe after many hours of off cycle things change. Dont know the answer to that one.

Also it appears we are running with a SH in the range of 30 to 40C (nominal is 35C) whereas the quoted SH for the compressor is 20C at the rated load.

Gary points out this will cause high discharge temperatures (which we do indeed get) and this in turn is a possible cause for failure.

Poor oil return (unproven but possible) and egg frying hot - looks like a recipe for short lived compressor.

Thanks again for your information.

Chef

[mad fridgie]

RLK - many thanks for your info.

The system has a low pressure switch but is normally always manually controlled, even so it does drop out if the suction goes past 0PSI. The high pressure switch is set to allow initial pulldown from warm and is about 225PSI.

It is water cooled and the condenser seems fine as the liquid out is just a few degrees above water temp.

The unit is a TM13 which is similar to a Sanden SD5. It is rated at 14,000BTU at 1800RPM for HBP applications and we calculate we are getting about 11,000 to 12,000 BTU at our conditions. The eutectic capacity is 8.6 USgal in all 4 plates

The max BTU acceptance of the plates is unknown at the moment as the coil arrangement inside the plate is unknown and some are sealed so we can never open it to find out. The only way would be to measure the pipes internal volume once the system is apart but it is an approximate method only.

The compressor failure by oil hold up is possible but whenever we have checked it the oil quantity seems OK for the unit - normally we test about 2 or 3 oz in the compressor which is pretty close to the specs for an SD5. However we always test the oil immediately after a cycle so the unit should have the correct oil returned to it and maybe after many hours of off cycle things change. Dont know the answer to that one.

Also it appears we are running with a SH in the range of 30 to 40C (nominal is 35C) whereas the quoted SH for the compressor is 20C at the rated load.

Gary points out this will cause high discharge temperatures (which we do indeed get) and this in turn is a possible cause for failure.

Poor oil return (unproven but possible) and egg frying hot - looks like a recipe for short lived compressor.

Thanks again for your information.

Chef

Not sure your calcs are correct, at rated 7.2C SST, you get 14000BTUs, but you are running at -20C SST, you would be lucky if you are getting 5000BTUs,
Your total load presuming 100% water (of course is not) would be 16100 BTU (allowance in SHC, to cover cabinet)
You said pulldown time 2.5hrs, so averaged duty is 6440 BTU, you would expect this as SST would start higher then drop towards the end, to below 5000BTUs
Very high discharge temp has also been caused by the introduction of the suction to liquid line heat exchanger. ( addition SH on and above what is leaving the cabinets)
It was indicated that major changes were not applicable to this system.
If that is not the case, then 2 seperate TEVs and 2 liquid sol valves, fridge off on plate stat. The temperature of the refrigerant is not important?
The freezer sol valve, off when comp goes off (stops liquid migration) Do not worry about pump down, as trying to set a very low differential on this application would be difficult. Because you are using aphase change, OFF cycle pressure will remain constant. Only when completly melted will pressure rise, to late!

[Chef]

Not sure your calcs are correct, at rated 7.2C SST, you get 14000BTUs, but you are running at -20C SST, you would be lucky if you are getting 5000BTUs,
Your total load presuming 100% water (of course is not) would be 16100 BTU (allowance in SHC, to cover cabinet)
You said pulldown time 2.5hrs, so averaged duty is 6440 BTU, you would expect this as SST would start higher then drop towards the end, to below 5000BTUs

I calculated the cooling using the volumetric flow rate of 0.0042m3/hr and a density of 5.1Kg/m3 to get mass flow of 0.021Kg/s.
Then from the PH diagram get H3 and so H4 and also H1 giving a deltaH in the evap of 150KJ/kg.
Kw=mass flow * deltaH = 3.15KW or 10,754BTU

Dont forget that the rated duty is air cooled condenser and discharge of 16bar so when water cooled the discharge pressure drops and the H3 moves to the left giving more cooling than you can deduce from the curves alone.

Pulldown time as I call it is from warm ie room temp and cycle time the period to cool from cold to very cold. The first is 2.5 hours and the second is about an hour.

Not knowing how much ice is still left in the plate when the cycle time is measured does not really give an accurate measure of the duty.
Anyway if all the ice was to be refrozen then 32litres of water at 333j/g yeilds 10.6Mj and at 3.1Kw that would yake 59 minutes so I think the numbers are quite correct but please check them and let me know if got someting wrong.

Chef

[mad fridgie]

I calculated the cooling using the volumetric flow rate of 0.0042m3/hr and a density of 5.1Kg/m3 to get mass flow of 0.021Kg/s.
Then from the PH diagram get H3 and so H4 and also H1 giving a deltaH in the evap of 150KJ/kg.
Kw=mass flow * deltaH = 3.15KW or 10,754BTU

Dont forget that the rated duty is air cooled condenser and discharge of 16bar so when water cooled the discharge pressure drops and the H3 moves to the left giving more cooling than you can deduce from the curves alone.

Pulldown time as I call it is from warm ie room temp and cycle time the period to cool from cold to very cold. The first is 2.5 hours and the second is about an hour.

Not knowing how much ice is still left in the plate when the cycle time is measured does not really give an accurate measure of the duty.
Anyway if all the ice was to be refrozen then 32litres of water at 333j/g yeilds 10.6Mj and at 3.1Kw that would yake 59 minutes so I think the numbers are quite correct but please check them and let me know if got someting wrong.

Chef

Moving imperial to metric, (working imperial as you uses BTU and gals)
You do not know how frozen the plate is when you restart the machine. But we do know the state when you start the system.
max weight of water 68LB (not quite true as there will be a minimum 7% air gap to allow for expansion, and reduction for volume of refrig pipes)
Start 100F 1BTU/Lb/F
Freezing Point 32F (not quite true but close enough) 144BTU/F (in real lfe is over varing temps) and would be less as contains an anti freeze
Final 0F 0.5BTU/Lb/F
so 68* 68= 4624
68*144=9792
68*32*.5=1088
Total=15504BTU (rounded up to 16100BTU, in my head lol)
Run time 2.5Hrs
Therefore averaged hourly duty 6440BTU (1.88KW)
At the start SST will be high, so actually duty is likely to be higher than the rated 14000BTUs, dowards the end duty i suspect is below 3000BTUs.
Yes you are correct your SCT is lower as well as the sub-cooling, but your SH is whey higher than design, (there is a bit of to and fro)
Your compressor calculations do not account for volumetric efficiency.(which is an unknown)

[Chef]

Start 100F 1BTU/Lb/F
Freezing Point 32F (not quite true but close enough) 144BTU/F
Final 0F
Start 100F 0.5BTU/Lb/F
so 68* 68= 4624
68*144=9792
68*32*.5=

Well the plate freezes at -20C so we also need to account for that drop in temp and the boxes also have some thermal inertia that should be accounted for and The Grande Fromage will always be adding some food in the fridges and freezer as well but not too much.

So when I add those numbers up I still get the same result for the cooling effect.

Chef

[Chef]

Your compressor calculations do not account for volumetric efficiency.(which is an unknown)

OK missed the last part of your calcs post - now I see it all.

Your right about the efficiency so if that is factored in I get 2.3Kw which is getting close to your 1.9Kw so it seems we can agree somewhere around this position.

But it is useful to hammer it out and get closer to the real values as it will be needed later to spec out a new compressor if thats the way it finally goes.

Thanks MF

Chef

[Gary]

The freezer sol valve, off when comp goes off (stops liquid migration) Do not worry about pump down, as trying to set a very low differential on this application would be difficult.

Good idea... but the solenoid valve should be in the common liquid line, preventing migration to all of the plates.

[Peter_1]

http://www.refrigeration-engineer.co...+liquid&page=3 post 47