Cold Room Pressure

[Lauren]

Dear All,

I need help for the following question and thanks in advance,

“how do we determine pressure inside the cold room assuming the cold room is at sea level, and 101.3 kPa. The cold room is operating at -20 deg C. Would room pressure be affected by fan coil defrosting?”

Thanks,
Lauren

[sachin230]

Are you aware of forum rule for multiple thread

[NoNickName]

Unless the room is air-tight, the pressure in the room is exactly the same than outside of it.

Don't post the same question over and over.

[Aik]

Unless the room is air-tight, the pressure in the room is exactly the same than outside of it.

Sorry, I disagree with your opinion.
In air-tight room with the temperature -20 C pressure in room will less than out of the room where temperature is about 20 C.
On account of thise, in cold storage install equalizing valve.
Air pressure you can find:
p=ro*R*T/M
p - pressure of air [Pa]
ro - density of air [kg/(m^3)]
R - universal gas constant = 8.31
T - temperature of air [K]
M - molecular mass of air =0.028 [kg/(m^3)]

[Sridhar1312]

Cold room will under negative pressure compared to outside as we do not take any fresh air and gets infiltrated by door opening or closing.

[Tesla]

The pressure would vary throughout the room due to air motion from the fan and turbulence through coil. Also vapor pressure deficit would have an effect from moisture removal via defrost. It can be calculated from psychometric chart with Pressure Volume Temperature relationship. Myth Busters did an interesting demonstration on pressure/weight in a truck with with pigeons. On average as Sridhar1312 wrote above is correct.

[NoNickName]

Exactly, that's why I said "UNLESS the room is air-tight".

[icecube51]

Exactly, that's why I said "UNLESS the room is air-tight".

some youngsters are so eager to replay that they don't take the time to read the questions fully.

they are young and still learning, we have to forgive them.

Ice

[frank]

Sounds like a college question to me.

[Lauren]

Sorry as I am new to RE, will be careful next time and understand the rule...
Sorry for any inconvenience caused..

[mad fridgie]

A coolstore should a have pressure relief vent, that being the case the static pressure is the same. (as stated by nonickname)
You will however get velocity pressure changes within the room (as stated by tesla)
After defrost, well that down to how the system is set up, but lets say poorly, "no fan delay or drip down time" fan starts with a hot coil, then a large pressure increase occurs through the coil "cold air expanding while being warmed" Boyles Law????? along with a velocity pressure wave.
The more inresting one is a chest freezer, open the lid, then close quickly. leave a few seconds, try to open again. If the freezer has agood seal you can not open.
The warm air is cooled (reducing in volume) causing a vacuum inside.

[Lauren]

Dear All,

Thanks for yr reply.
But based on our cold room design, we have 3 evaporators and each time defrosting (with hot gas at 45 deg C) take 30mins with fan is turn off for only one evaporator and the remaining 2 evaporators still operated as usual to maintain the cold room temperature at -20 deg C. Between, fan delay also applied.
We also have a vent port to equalize the cold room pressure.

With such design, would the cold room pressure still be affected by fan coil defrosting?
How much different if compared with outside pressure and cold room pressure at the same sea level?

I did some calcualtion and found out the cold air(-20 deg C) density is higher than normal air (33 deg C) density... hence the cold room air pressure is greater than atmospheric pressure (14.7 Psia).

Please kindly advise me. Many thanks.

[mad fridgie]

Dear All,

Thanks for yr reply.
But based on our cold room design, we have 3 evaporators and each time defrosting (with hot gas at 45 deg C) take 30mins with fan is turn off for only one evaporator and the remaining 2 evaporators still operated as usual to maintain the cold room temperature at -20 deg C. Between, fan delay also applied.
We also have a vent port to equalize the cold room pressure.

With such design, would the cold room pressure still be affected by fan coil defrosting?
How much different if compared with outside pressure and cold room pressure at the same sea level?

I did some calcualtion and found out the cold air(-20 deg C) density is higher than normal air (33 deg C) density... hence the cold room air pressure is greater than atmospheric pressure (14.7 Psia).

Please kindly advise me. Many thanks.

Please read what you have written, you have the answer.
I have given you a small clue

[Aik]

Exactly, that's why I said "UNLESS the room is air-tight".

Sorry, don't see "UNLESS"

some youngsters are so eager to replay that they don't take the time to read the questions fully.

they are young and still learning, we have to forgive them.

icecube51, thanks for yours understanding.

I did some calcualtion and found out the cold air(-20 deg C) density is higher than normal air (33 deg C) density... hence the cold room air pressure is greater than atmospheric pressure (14.7 Psia).

Besides density also there is temperature of air... Check your calculations...

[Lauren]

Hi,

Here is my calculation,
1) Air at -20 deg C;
Density air = 1.4135 kg/m3
Temp= 253.15 K
Air constant, R= 286.9 J/Kg.K
P= density x temp x air constant
P= 1.4135(253.15)(286.9)
P= 102.66kPa
P= 14.9 Psia

2) Air at 33 deg C;
Density air = 1.154 kg/m3
Temp= 306.15 K
Air constant, R= 286.9 J/Kg.K
P= 101.36kPa
P= 14.7 Psia

Please kindly advise me if you think my calculation is inaccurate. Many thanks

[NoNickName]

please read what you have written, you have the answer.
I have given you a small clue

rtoflmao :d

[Lauren]

Hi Mad Fridgie,

I got what u meant but recently I encountered a case said that my cold room pressure is muiltiple times greater than outside almospeheric pressure with the mentioned design...

I have no concrete pressure data in cold room at -20 deg C, hence want to seek you guys expertise on this.

After calculated the air pressure for cold room and normal temperature and found out that the air pressure slightly higher in cold room altho knowing that density in cold room is higher than normal air temperature.

Thanks

[mad fridgie]

Maybe you should indicate your problem, as we all seem to be struggling with what you are asking.
This then may make more sense.
Yes cold air is more dense, so the pressure at the bottom of a static head level will be higher than the ambient.
Think like a cup holds the static head, if you continue to fill the cup it will overfllow so at the top the pressure is equal to the ambient

[Aik]

Hi,

Here is my calculation,
1) Air at -20 deg C;
Density air = 1.4135 kg/m3
Temp= 253.15 K
Air constant, R= 286.9 J/Kg.K
P= density x temp x air constant
P= 1.4135(253.15)(286.9)
P= 102.66kPa
P= 14.9 Psia

2) Air at 33 deg C;
Density air = 1.154 kg/m3
Temp= 306.15 K
Air constant, R= 286.9 J/Kg.K
P= 101.36kPa
P= 14.7 Psia

Please kindly advise me if you think my calculation is inaccurate. Many thanks

If you have air-tight room you put in thise room warm air, close door and cooling air.
If room air-tight than mass of cold and warm air be the same (volum of room also the same).
Density of cold and warm air is the same, because mass and valume is the same (ro= mass/volume of room)
Than:
at 33 C and 40% rh density = 1.12 kg/(m^3)
p=1.12*8.31*306.15/0.028=101.8 kPa
at -20 C
p=1.12*8.31*253.15/0.028=84.1 kPa

[Lauren]

Pardon me for my ignorance, so may I presumed that you are agreed with my cold room pressure calculation which cold room pressure at -20 deg C is greater than ambient pressure with my current cold room condition?

And the cold room pressure wouldnt be affected much with fan coil defrosting?

Thanks for your expertise reply and comments.

[Aik]

Pardon me for my ignorance, so may I presumed that you are agreed with my cold room pressure calculation which cold room pressure at -20 deg C is greater than ambient pressure with my current cold room condition?

No, no and once more NO...
In air-tight cold room after cooling pressure will be 84.1 kPa.
Ambient pressure (at 33 C) is 101.8 kPa.
If there isn't vent port which equalize cold room pressure than at this temperature you may don't open the door.
You have vent port in your room, than pressure in room = ambient pressure... many peple tell you about it in this theme

[NoNickName]

Pardon me for my ignorance, so may I presumed that you are agreed with my cold room pressure calculation which cold room pressure at -20 deg C is greater than ambient pressure with my current cold room condition

You must be challenged. I fully sympathise with you.

[Lauren]

Hi Aik,

Sorry as I missed out yr reply as I was posting my query before seeing yr reply.
Thanks for highlighting me regarding the densiry air...

[Lauren]

Hi Nonickname,

Is fine with me as I'll treat this as my learning path. I am a new bird in refrigeration field... No pain no gain....(",)

Thanks all for the guidance.

[Segei]

I'm just wandering. How can we design(and build) air tight cold room? Usually, this room has evaporator and evaporator has a drain and drain goes outside of this cold room.

[NoNickName]

I'm just wandering. How can we design(and build) air tight cold room? Usually, this room has evaporator and evaporator has a drain and drain goes outside of this cold room.

With a peristaltic condensate pump and a non return valve

[Segei]

With a peristaltic condensate pump and a non return valve

Definitely, you can do that. But you need a water strainer... May it is better water trap. You don't need pump, strainer, non return valave, vent port....

[Lauren]

Thank you all for the expertise comments...
I will close this thread now....

Lauren

[Lauren]

guess I will wait for 1 more day to make sure that "Segei" & "nonickname" have end their discussion here...
Thx.

[ref717]

He definitely answered his own question.
All industrial aircoolers have drain pipes that is connected down to the waste pipe via P-traps to prevent foul air from going back into the rm. and keep the pressure atmospheric.
Some cold storage rms. especially blast freezes have vacuum breakers inside to prevent vacuum pressure as what mad fridgie explained.

[NoNickName]

Definitely, you can do that. But you need a water strainer... May it is better water trap. You don't need pump, strainer, non return valave, vent port....

None of that is needed. Or am I missing something about your post?

[Segei]

None of that is needed. Or am I missing something about your post?

My point is that drain of typical evaporator will equalize the pressure inside and outside.