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samiam
09-07-2008, 09:40 PM
Hi all,

I need to pick some brains.

I have to create a large coldroom (to be operated at 0C) and have selected the evaporator with the following:

3 off evaporators
25 kW duty
Propylene glycol (35%) Supply/Return at -10C / -6C
Air on / Air off at 2C / 0C
2off 800 diameter fans (100 Pa) per unit
From the above evaporators can be selected.

Here comes my Story/Question: During the early project lifespan, the Coldroom shall be used for other purpose and operate at 20C (air off). However, the 3 evaporators must not change when changing from a 20C room to 0C room.

From my calculations - when operating at 20C, the evaporators are physically oversized as the LMTD differ greatly.

Any tips?
Does this mean that my valve station will be modulating alot?
Has anyone done this before.
Whats the dehumidification effect?

Regards,

Josip
09-07-2008, 10:04 PM
Hi, samiam :)

.... what is your cold room for... onion .... potato... something else ...

Best regards,Josip

samiam
09-07-2008, 10:24 PM
Hi Josip,

Its a general coldroom - no product spec yet.

Regards,

US Iceman
09-07-2008, 11:25 PM
First of all that is a diverse range of temperatures you have. One main issue I see initially is the need to provide uniform air flow of the area to be maintained by temperature.

Is this area constant during the operation at 20C? And, is this the same area that will be maintained at 0C? If so, then you need to maintain a uniform air flow throughout the space, which eliminates the possibility of of only using one or two coils at 20C as opposed to three coils at 0C.

When the space temperature is 0C the coil will probably operate with frost on them. During the operation with 20C the coils will be operating wet. Therefore, you either need to ensure the face velocity of air off of the coils is low enough to prevent water being pulled off of the coils. This could be done with VFD's or by carefully selecting the coils so that operate with low face velocity in either condition.

Then...you can evaluate the need for control valve modulation, floating set points for temperature control, etc.

And, just as important is the need to have the refrigeration system designed so that it can operate at both conditions (20­C or 0C) with the most efficient mode to deliver the lowest operating cost.

It's not impossible...it just takes some work and careful review.

NoNickName
10-07-2008, 07:10 AM
Keep the fans running on all evaporators, but let just one actually working. The total air flow will allow uniform air mixing, but the cooling capacity will just be 1/3.
Please note that the compressors or condensing unit that is, must be capable of working at 1/3 of design load.

samiam
11-07-2008, 04:28 PM
Hi - thanks for the feedback thus far.

A point that I forgot to mention. The room is a 0C Coldroom but until they build the reminder of the warehouse (in 3 years time). Until then it needs to operate at 20C.
(I do have VSD fans)

any more tips

Magoo
29-09-2008, 11:24 PM
Look at using water cooling through evaps and a dry cooler, you know the wet buld condition, if necessary add a high temp water chiller to suppliment cooling capacity, later on design a chilled glycol system for total duty once at 0'C, don't forget partial defrost on evaps.
magoo