Cold Room Design

[vijay_study]

Dear All,

We have recently designed a cold room system in order to maintain Temperature = 4 - 6°C and Relative Humidity = 50 ± 5%.

We are achieving the conditions during most part of the day however we have problems during following situations:

1. DEFROST CYCLE:
During the defrost cycle the Rh is shooting upto 70% Rh which is not acceptable.

Defrost cycle is set for 3 minutes every hour.

2. COMPRESSOR CUT OUT:

During compressor cut off which is set at 4°C again the Rh is shooting upto 70%.

The above increase in relative humidity is not acceptable to the client even though the increase is only for 5 to 8 minutes in a hour.

Please give us your valuable suggestions regarding how to short out the problem.

Regards

vijay_study@rediffmail.com

[NoNickName]

Compressor must never stop, and correction of temperature to be done via electric heaters of the same capacity, and controlled with triacs.

Defrost must be prevented by using a saturation sensor, which will bypass hot gas when evap temp is below the frost point

[Peter_1]

A temperature change from 4 to 6°C with the same absolute humidity gives already a 6%RH change.
This phenomena is the result of a nature law which you can't change at all.

So the first thing you have to take care of is to control your temperature much narrower.

Finetune afterward the RH with electrical heaters, preferable controlled with a PID algorithm.

If you don't want a change at all, you only can achieve this with a proportional control of cooling and heating.

Is it a DX evaporator or a water coil?

What's the product in the room?

Is it often opened?

Why you need anyhow a defrost with a temperature of 4 to 6°C, especially with these low RH?

[US Iceman]

This seems like the evaporating temperature was selected well below 0C to provide dehumidification in an attempt to reach 50% RH.

When you start to approach evaporating temperatures below 0C for environmental chambers (this is what you are trying to accomplish, this is not a normal cold room situation) you can have these problems.

The system must never be shut down, or if it is, then you need redundant refrigeration systems.

You may have to treat the problem as multiple systems.

1. The refrigeration systems provides the low temperature.

2. Reheat controls the discharge air temperature to the tolerance you need.

3. Humidity injection or desiccant systems may be required to fine tune the air condition.

The temperature of the evaporator coil must operate above the air dew point temperature or frost will form.

[Brian_UK]

I also wonder where the temperature and humidity sensors are located in relation to the cooling coil.

[Andy]

Hi Vijay

the only way I can think of acheiving constant RH is to fit two evaporators, run one defrost one, then run the other.

Kind Regards. Andy

[sunilbhat]

Dear All,

We have recently designed a cold room system in order to maintain Temperature = 4 - 6°C and Relative Humidity = 50 ± 5%.

We are achieving the conditions during most part of the day however we have problems during following situations:

1. DEFROST CYCLE:
During the defrost cycle the Rh is shooting upto 70% Rh which is not acceptable.

Defrost cycle is set for 3 minutes every hour.

2. COMPRESSOR CUT OUT:

During compressor cut off which is set at 4°C again the Rh is shooting upto 70%.

The above increase in relative humidity is not acceptable to the client even though the increase is only for 5 to 8 minutes in a hour.

Please give us your valuable suggestions regarding how to short out the problem.

Regards

vijay_study@rediffmail.com

dear vijay

In my view you should put off evaporator fans during defrost and compressor off cycle. this may help to avoid excess rh
sunil bhat
sunilbhat1@hotmail.com

[old gas bottle]

had this before, think the guys are telling you your kits not up to it,if its a single evap/condenser every time the compressor stops for whatever reason the RH wil shoot up,in fact at night when the unit is left shut and unatended the off cycle is often a lot longer and then the RH goes through the roof, you could try shortening the cut in /out diff as close as you can , use off cycle defrost [just the fans no heaters], put a good size dehumidifier in the room[plasteres rental size], if it works install a perminent one, theese are only quick fixes to prove a point, as previousley stated, big upgrade needed.

[olumide]

Hello everybody,
I am highlly intressted in cold room designing,but am about sitting for my NVQ2 examination by june as Refrigeration and Air-condition engineer,I will be glad if anbody can help to achivied my dream.

[Samarjit Sen]

The thread is an old one, but as I viewed it now, I thought that may be I could suggest something in case the problem is still persisting.

The Room Temp. is 4 to 6 Deg. C with a RH of 50 %. Firstly, the evaporator should have a air defrost system. The Fan should run. Defrosting every hour for 3 mnts is not required. In such case the defrost would be for 5 to 10 mnts every 6 hours.

What was the product loaded and what about the door service. What is the coil details such as fins per inch, air flow etc. Are these coils locally fabricated or are of standard manufacture.

[janubas]

Hi,

B4 getting into the probs, first let me be explained why do you need a defrost for such a positive temp

[janubas]

vijay, giv more details about the product u r storing. lets know if all the fundas are right