Problematic Economiser

[Kevin Yeo]

For a supermarket project, we have used 2 nos of 75 hp Screw compressor for Low Temp system. [Part of the schematic layout is per attached].
It’s a water cooled system, R22, with individual economiser.

The problem I faced is the frequent breakage of economiser / subcooler.

As the previous designer had resigned, I can only manage to show the layout based on site layout. Outgoing liquid temperature is set as +10 deg C. Beyond that, SV at economiser would de-energise to stop sub-cooling of liquid, no liquid flow for TXV. It would also mean no ECO cooling for screw compressor.

Economiser is one to one with compressors and there’s no common header for it. When pre-set temperature of +10 deg C is reached, no subcooling and no ECO. Could it be attributed to the failure of subcooler?

When SV de-energise, subcooler temperature = warm liquid temperature. Pressure in ECO line almost vacuum.

When SV energise, subcooler temperature = +5 deg C or lower.

Thermo cyclical changes…

Any comment on this system and any creative ideas to solve this problem? Thanks all

[ecclesk]

The problem I faced is the frequent breakage of economiser / subcooler

What exactly do you mean by this?

I would have thought that the SV feeding the TXV should be energised when comp is running and the temp of the subcooleing should be controlled by the TXV.

What controls the SV on liquid outlet of subcooler?

[Kevin Yeo]

SV at liquid outlet energise when minimum of 1 compressor runs.

Economiser has cracked 2 times since installation last year. Product defect should be ruled out. So I'm not sure if its the system design problem.

Liquid outlet temperature is controlled using Saginomiya thermostat and set at +10 deg C. TXV operation comes in as temperarature control but not beyond +10 deg C as SV feeding to TXV would close when preset temperature is reached.

[ecclesk]

Have seen a similar problem before, fairly new installation and the subcooler had ruptured internaly, turns out that it was down to the engineering staff on site, whenever they isolated a compressor for maintenance they were closing every valve around the compressor & subcooler and by doing this they were trapping pure liquid inside the coil of the subcooler, when the temp in the plant room increased liquid in the coil had nowhere to expand and ruptured the coil.

[PobodysNerfect]

I can understand you close the solenoid valve for the sub cooling when preset temperature is obtained, but why do you also close the solenoid valve in the outlet from the economizer (liquid for the TXV)?

[dallan]

What happens to the SV feeding the TEV when the compressor is off. If compressor 1 is off does the SV feeding Subcooler 1 stay energised.

The control logic has to be changed. I would remove the outlet SV and have the SV which feeds the subcooler tev in line with the thermostat and the compressor. I would also consider removing the thermostat, you may have slight problems with the evpansion valves being oversized but you will reduce the operating costs.

How are the compressors staged to operate.

David

[Kevin Yeo]

SV at the subcooled liquid outlet serves as protection. During power failure which means no compressor is running. SV at subcooled liquid would close to prevent liquid flushing back.

David,
I think wat you said is correct, the thermostat was added subsequently. Perhaps the TXV/ economiser is too big. I've checked the mass flow for ECO required per compressor to be 373 kg/h at -37 deg C/ 40 deg C SST/ SCT. Now waiting for Alfa Laval supplier to feedback on this PHE size.

Do you guys think that trapped pure liquid (around +40 deg C) that expands and contracts when TXV in operation cause the rupture of subcooler like wat ecclesk is saying? A cyclical changes in temperature of subcooler. Perhaps TXV needs to be operation unless the particular compressor is not running. In this case, internal temperature of economiser is more stable. Any suggestion?

[dallan]

As detailed previously the tev feeding the subcooler should only be in operation if the relevant compressor is running.

A combination of this and the subcooled liquid SV is causing your problem, the exit SV should be removed. Why is there a concern about liquid flooding back, what do you imagine happening.

What type of pack/case controls are you using.

[chillyhamster]

Looking at the schematic, 1) should the liquid solenoid valve not be on the inlet to the plate heat ex, this would prevent liquid expansion on off cycle, 2) could you not install an evaperator pressure regulator in the economiser suction to limit the sub cooling effect to +5 deg C, would also act as a nrv.

[Kevin Yeo]

Dear all,

Sorry for my limited knowledge, may I ask the correct method of sizing plate heat exchanger for compressor. In this case, we used 2 x HSN7471-75. Eco mass flow req : 373 kg/h (-37 deg C/40 deg C), pressure 2.8 bar. Subcooler load required : 16.13 kW, evaporating -16.4 deg C. R22 refrigerant, water cooled.

Should i just use these 2 parameters to select a PHE?
And use 16.13 kW @ -16.4 deg C to select an expansion valve?

Thanks in advance...