R404a water cooled condensing unit with 8oC cooling water

[Peter_1]

What capacity do you need? Pure water? What are your water connections? Seems you're confusded, only a water regulating valve like WVFX. See it as a proportional control of speed on condensor fans.

[wta12]

What capacity do you need? Pure water? What are your water connections? Seems you're confusded, only a water regulating valve like WVFX. See it as a proportional control of speed on condensor fans.

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As shown, i need est 6 kw come along with pure water (softened chilled water). Will this valve be help at the start-up stage, when we can help low condensing pressure condition ?
@all : i still have question about 0 oC or less suction gas temperature with r404a and piston compressor. Normally suction gas temp at 20 oC as standard. What problem will come when we have lower than standard suction gas temp ?

[nike123]

Superheat is required to make sure that liquid refrigerant cannot get in to the compressor.

If you have correct superheat, than suction gas temperature could be as low as evaporation temperature + evaporator (useful) superheat value. In air-conditioning, with TXV as expansion device, suction superheat can be as low as 5K (R404a) without any problem. In freezers superheat can be as low as 2K

In closely coupled devices as Air-to water heat pump or similar, there is not much un-useful superheat between evaporator and condenser, and having there total superheat as big as 15K will make it impossible, if suction line heat exchanger is not used.

Anyhow, device designer responsibility is to make check to be sure that suction accumulator or suction line HE is not required for that low superheat.


http://www.refrigeration-engineer.co...p/t-18295.html

In your case, you probably have suction line HE and that can rise your suction line temperature enough that there is no need for suction accumulator.
If your discharge temperature is to low, than high side of suction line HE is not heating enough suction line gases, and your suction temperature and suction superheat is to low. Therefore you must maintain condensation temperature at design level (which is usualy around 40°C) to ensure designed cca 10K superheat at suction line HE. With 10K superheat at any time there is no danger to liquid in compressor and therefore no requirement of 20°C liquid line temperature.

If you follow that rule blindly, at -25°C evaporation, you will be having 45K suction superheat, which will make discharge gases hot as hell.

[Peter_1]

... Therefore you must maintain condensation temperature at design level (which is usualy around 40°C) to ensure designed cca 10K superheat at suction line HE. ....

Nike, I disagree with this phrase - perhaps I misunderstood - but what about floating HP? SH is maintained even with floating HP.
I think WTA12 will become even more confused now, so don't listen to me now WTA12.

[nike123]

My reasoning is as folows, but maybe, I am wrong. I am not that god in theory.

Capacity of suction line HE depend on TD of that heat exchanger. If we have low discharge temperature, than TD of suction line HE is lowered, and therefore achieved superheat/subcooling is also lowered.

Nominal SLHE selections should be based on system horsepower, which will provide a nominal 10ºF subcooling and a nominal 20ºF suction superheat for R-22, R-502, R-404A, R-507, R-407A, R-504, R-134A, R-407C and R-410A at 110ºF condensing and 25ºF suction. For example, for a 1-1/2 HP system, select an SLHE 1-1/2. Do not select undersized models to avoid high suction pressure drops and gas whistling.

Selections based on suction and liquid line sizes are appropriate for standard low, medium and high temperature refrigeration systems provided the system HP rating closely matches the SLHE selection.

Selection of oversized models will provide additional liquid subcooling and suction superheat. Oversized models are acceptable only if the suction gas temperature to the compressor is not raised above 65ºF for proper compressor performance.

http://heatexchangers.doucetteindust...at-exchangers?

[wta12]

Superheat is required to make sure that liquid refrigerant cannot get in to the compressor.

If you have correct superheat, than suction gas temperature could be as low as evaporation temperature + evaporator (useful) superheat value. In air-conditioning, with TXV as expansion device, suction superheat can be as low as 5K (R404a) without any problem. In freezers superheat can be as low as 2K


Anyhow, device designer responsibility is to make check to be sure that suction accumulator or suction line HE is not required for that low superheat.


http://www.refrigeration-engineer.co...p/t-18295.html

In your case, you probably have suction line HE and that can rise your suction line temperature enough that there is no need for suction accumulator.
If your discharge temperature is to low, than high side of suction line HE is not heating enough suction line gases, and your suction temperature and suction superheat is to low. Therefore you must maintain condensation temperature at design level (which is usualy around 40°C) to ensure designed cca 10K superheat at suction line HE. With 10K superheat at any time there is no danger to liquid in compressor and therefore no requirement of 20°C liquid line temperature.

If you follow that rule blindly, at -25°C evaporation, you will be having 45K suction superheat, which will make discharge gases hot as hell.

I checked selection condition. We always have 10oK superheat (will be set by TXV) : -30oC To(evaporation) + 10 oK superheat = - 20 oC suction gas temp. and -10oC To(evaporation) + 10 oK superheat = 0 oC suction gas temp..
Evaporator also chosen according -10oK superheat

Other Question:
1.In case we have shell and tube condenser, which may act as receiver also. should we have dedicated receiver for my case (r404a with low condensation temp) ?
2. Crankcase heater should be installed with low suction gas like this or not ? Normally we need not have this one in our country ever after long time turn back cause high ambient condition (lowest 8oC- normally 20-25oC) but i'm not sure if compressor and condenser inside room (20-28 oC).

Regards

[Peter_1]

I checked selection condition. We always have 10oK superheat (will be set by TXV) : -30oC To(evaporation) + 10 oK superheat = - 20 oC suction gas temp. and -10oC To(evaporation) + 10 oK superheat = 0 oC suction gas temp..
Evaporator also chosen according -10oK superheat

Other Question:
1.In case we have shell and tube condenser, which may act as receiver also. should we have dedicated receiver for my case (r404a with low condensation temp) ?
2. Crankcase heater should be installed with low suction gas like this or not ? Normally we need not have this one in our country ever after long time turn back cause high ambient condition (lowest 8oC- normally 20-25oC) but i'm not sure if compressor and condenser inside room (20-28 oC).

Regards

Don't understand why you subtract twice 10K and uses twice evaporation. When refrigerant is leaving your evaporator, it's all evaporated as a superheated gas.

You evaporate at -30°C, 10 K superheat is too much, 7K, so -23°C leaving evaporator and +/- 10K along suction line = -13°C inlet compressor

Select compressor for -30°C and 20K total SH and your evaporator at -30°C and 7K SH.

1. You don't need a receiver but charge is then somehow critical. You can install one but it isn't necessary.
And you will not see low condensing pressures, the WVFX will maintain a stable HP.
I feel you still doesn't get the whole picture.
Perhaps this will help http://www.danfoss.com/NR/rdonlyres/.../PFD00A102.pdf


2. If you have pump down, no CCH needed. As you said, it's hot enough in your country to omit a CCH. The temperature of the returning gas has nothing to do with the need for a CCH.

[wta12]

Don't understand why you subtract twice 10K and uses twice evaporation. When refrigerant is leaving your evaporator, it's all evaporated as a superheated gas.

You evaporate at -30°C, 10 K superheat is too much, 7K, so -23°C leaving evaporator and +/- 10K along suction line = -13°C inlet compressor

Select compressor for -30°C and 20K total SH and your evaporator at -30°C and 7K SH.

1. You don't need a receiver but charge is then somehow critical. You can install one but it isn't necessary.
And you will not see low condensing pressures, the WVFX will maintain a stable HP.
I feel you still doesn't get the whole picture.
Perhaps this will help http://www.danfoss.com/NR/rdonlyres/.../PFD00A102.pdf


2. If you have pump down, no CCH needed. As you said, it's hot enough in your country to omit a CCH. The temperature of the returning gas has nothing to do with the need for a CCH.

Sorry for my unclear statement. It's water cooled condensing unit for cold and chiller room.
First is for meat and sea food so they have -30 oC evaporation and -23 oC cold room temperature with suction gas temp -20oC for 10oK super heat

Second is a for higher condition (vegetable, beverage,....) so they have -10 oC evaporation and 0oC suction gas temp. (10o SH also) like compressor selection data This for worst case Maybe it's T evaporation will be raised up if have other requirement.

For SH calulation : for +10 oK along suction line as you said, could i avoid it because if suction line exposed to air room, we will have frost. i want to insulated it.

I have no experience with refrigerant line cause nearly all of my work before come with HVAC system so most of refrigeration work come from chiller's manufacture guys. and it's completely different condition with HVAC chiller. May be it's not all but i'm very appreciated your supports.
Thank alot

[Peter_1]

Take the hottest temperature you will ever see for the water supply, add 5 to 7 K and use this to calculate your compressor and your expansion valve.
Regulate your valve in winter for the hotter conditions you will see in summer and it will save you a lot of energy.
And, your HP will be stable the whole year long.

[wta12]

Take the hottest temperature you will ever see for the water supply, add 5 to 7 K and use this to calculate your compressor and your expansion valve.
Regulate your valve in winter for the hotter conditions you will see in summer and it will save you a lot of energy.
And, your HP will be stable the whole year long.

Please clarify for me the Hotter condition is the HP - discharge gas condensing temperature, Isn't it ? and why we need higher in winter cause i use chilled water with extremely stable 8oC (12 oC max but seem to never happen) ? All of my unit with cold room inside building so weather have very tiny affect.
Regards