PDA

View Full Version : Three Evaporators, three different evaporation temeperatures







MilosBog
12-09-2016, 01:57 PM
Hi Guys,
I have 3 different evaporators - temperatures are -18C, -10C and 0C Evaporation.

I will place non return valves and the pressure controllers on the evaporators with lower pressure.

The question would be: what should be taken as evaporation temperature for sizing the compressor? I could easily calculate what would be the temperature of mixed fluids (together with pressure) and i could take that as evaporation temperature but just wanted to know is that the right way.

how is this sorted with VRF systems?

BR

mbc
12-09-2016, 02:40 PM
Hi
As you know you should set your suction in lowest temp .
but you can not choose all room load to calculate compressor load if it has not unloading system.
or your compressor should be able works with VFD from lowest load and cover your total load

Why ?
because when two of them those not need to works you suction pressure goes to low .
I think it is better you choose ;
total load/2

you need 2 evaporator pressure ( back pressure ) with to check valve and 3 solenoid valve and all
of them should be have one timer for defrost with 3 contactor / one for each room
with thermal control of defrost .

I would say do one unit for each one if your system not to small .

Josip
12-09-2016, 03:57 PM
Hi, MilosBog :)


Hi Guys,
I have 3 different evaporators - temperatures are -18C, -10C and 0C Evaporation.

I will place non return valves and the pressure controllers on the evaporators with lower pressure.

The question would be: what should be taken as evaporation temperature for sizing the compressor? I could easily calculate what would be the temperature of mixed fluids (together with pressure) and i could take that as evaporation temperature but just wanted to know is that the right way.

how is this sorted with VRF systems?

BR

Your evaporation temperature/pressure should be around -23C or even lower (dt=5-7K lower then lowest evaporator temperature, in your case evaporator which works at -18C) ... back pressure controllers you can put only to systems with higher evaporation temp i.e. pressure otherwise will not work ...

We do not know anything about your system, but it will be much better to install two stage system (for sure more expensive, but definitely much better...)

Definitely your task is not simple.

Also for low temperature evaporator you need additional HE (economizer) to sub-cool liquid refrigerant coming from condenser before entering into low temp. evaporator ....

VRF system (if you are thinking about Variable Refrigerant Flow) cannot be used here ... your system is not HVAC ... it is refrigeration system and some other rules must be applied.

Best regards, Josip :)

MilosBog
12-09-2016, 06:21 PM
I have noticed now that I had German spell check whilst writing so I got tree instead of three. In such situations when I want to excuse for being lazy to check the spelling I write even more funnier sentences... Anyways Bok Josip and Salam Mbv and thanks for answering.

I am am bit reluctant to size compressor for the lowest temperature as that particular load is only 5% of all. But what I will try to do is to calculate evaporation temp when all three evaporator are working same time plus for some other cases and I will check out the compressor for all cases.

I wasnt thinking about using Variable refrigerant flow for this application, I was more interested in how they made it when some evaporator sate working with higher and some with lower temperatures

do you guys have more suggestions
br

mbc
12-09-2016, 06:22 PM
hi

the minimum evaporator temp. is -18

the 2-stage compressor limit start from - 25 Evap. temp.

I do not Know Why he can not use VDF ( variable Frequency Drive ) Could say more

I think he can use VDF By putting Pressure transmitter 4-20 MA and connect that to Analog port in VDF

and set VDF for that

by rising pressure in suction line speed of comp goes up .

setting should take a little more time

Magoo
13-09-2016, 02:03 AM
hi MilosBog.
calculate refrigerant flow rate for each systems operating at average design conditions. That will tell you the compressor swept volume capability required.

Josip
13-09-2016, 08:29 AM
Hi, MilosBog :)

Pozdrav!


I have noticed now that I had German spell check whilst writing so I got tree instead of three. In such situations when I want to excuse for being lazy to check the spelling I write even more funnier sentences... Anyways Bok Josip and Salam Mbv and thanks for answering.

I am am bit reluctant to size compressor for the lowest temperature as that particular load is only 5% of all. But what I will try to do is to calculate evaporation temp when all three evaporator are working same time plus for some other cases and I will check out the compressor for all cases.

I wasnt thinking about using Variable refrigerant flow for this application, I was more interested in how they made it when some evaporator sate working with higher and some with lower temperatures

do you guys have more suggestions
br

All is OK we catch almost all;)

After reading your answer ... My suggestion is to make all as per mbc suggestion for each system one unit or at least two systems ... one for low temp. system at -18C and another one for -10C/0C systems with back pressure regulating valve for system at 0C.

If you want to use only one compressor for all three systems your evaporating temperature/pressure at compressor suction should be at least equal or lower then -18C and of course you need compressor with capacity control when you have reduced load ... of course for that you can use compressor with VFD (variable frequency drive) but set point remain for the lowest requested temperature.

Note: Hi, mbc :).... I thought that VRF (Variable Refrigerant Flow) is related to:

https://www.google.it/search?q=variable+refrigerant+flow+daikin&rlz=1C1TEUA_enHR512HR512&espv=2&biw=1366&bih=648&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwi-18ig5YvPAhXCNhoKHWoJD68QsAQIag
or
https://en.wikipedia.org/wiki/Variable_refrigerant_flow

and not to VFD - variable frequency drive ...

Best regards, Josip :)

MilosBog
13-09-2016, 07:37 PM
Pozdravi Josip,greetings fellow Fridgies!
i will have Variable flow drive on compressor in any case so no worries about that.

I did some calculations for this for following scenarios:

low temperature evaporation runs only: compressor suits the needs
combined flows ( different evaporation temperatures streams mixing together) compressor is sized for this case
high temperature evaporation runs only : compressor has to work with 75% of capacity

i will place back flow preventers and for sure internal heat exchanger to stabilize the temperature on suction a bit.

Thanks to all of you guys!!!