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church2k
09-05-2007, 09:19 PM
Hi!, as i say in the title i need information about defrosting in industrial systems, but i,m not talking about roughly information(i know how the defrost system works), what i,m looking for is the wrote description of various defrosting systems, i would suppose to really explain what i need. For example, in a NH3 system:
Defrosting method:
1.- close the liquid valve for 2 minutes, for evaporate all the liquid inside the evaporator.
...and so...
i will feel grateful, because is an information that i really need.
Thank u all.

brian_chapin
10-05-2007, 12:31 PM
what i,m looking for is the wrote description of various defrosting systems, i would suppose to really explain what i need. For example, in a NH3 system:
Defrosting method:
1.- close the liquid valve for 2 minutes, for evaporate all the liquid inside the evaporator.
...and so...
i will feel grateful, because is an information that i really need.
Thank u all.

I'm not sure my NH3 liquid overfeed system is optimized but here is how the control logic is set up now.

1) Liquid Solenoid off - fans continue to run for 30 minutes. This is called pump-out

2) Fans shut off. Hot gas solenoid opened (small line) - a larger gas operated solenoid valve opens when the pressure reaches 40psi. This gas continues flowing through the coil (regulated at 70psi) for 20 minutes.

3) Hot gas solenoids close. Coil remains open to suction for 5 minutes. This is called the bleed time.

4) Liquid solenoid on - fans remain off for 5 minutes. This is called the cooldown time.

5) Fans turn back on - coil is back in normal operation.

Note: High temperature alarms at the coil are supressed for the entire defrost cycle + 20 minutes after the defrost.

The DX NH3 coils are in 35f rooms - so they simply close the liquid solenoid and keep the fans running to defrost the coil. This allows the room air to melt the frost off the coils.

NH3LVR
10-05-2007, 02:22 PM
1.- close the liquid valve for 2 minutes, for evaporate all the liquid inside the

Church2K;
You have stumbled upon a common misconception involving the pumpout of NH3 evaporators during defrost.
Defrost controls commonly are set for a 2-5 Minute pumpout on recirculated and flooded systems. In reality very little NH3 will evaporator in this short time (unlike ***** systems).
Brian mentions a 30 minute pumpout of his DX coils. This is more in line with the requirements.
This misunderstand carries over to attempts to control temperatures in flooded systems. I have seen flooded systems set up to control cold storages by cycling the accumulator feed. It does not work well, especially in one or two evap stems. Even in recirculated systems it can cause oscillation in the compressor room.

Sergei
10-05-2007, 04:24 PM
Brian described soft hot-gas defrosting. ASHRAE suggests this defrosting for coils over 15 TR capacity.
Pump out of liquid overfed(bottom) coils. Direction of suction line from this coil to main suction line is very important.
Horizontal suction line. In that case we have to evaporate liquid inside the coil. My tests have shown that after 5 min of pump out refrigeration load from this coil is going down. So, I prefer 10 min of pump out for this type of coils.
Vertical suction line. In that case we have to evaporate liquid from vertical suction line plus from coil itself. Very often it takes around 30 min pump out.

US Iceman
11-05-2007, 01:45 AM
Here is some information in the following links:
http://www.hantech.com/documents/PDF/technibriefs/tb9.pdf

http://www.hantech.com/documents/PDF/f100.pdf

http://www.irc.wisc.edu/ASHRAE-TC101/programs/Annual%202002%20Sluga.pdf

http://www.parker.com/RS/90-11a.pdf

http://www.hantech.com/documents/techbriefs/gasbasicsTB2_2-98.htm

I think the above cover most of the details.

church2k
02-06-2007, 11:15 AM
Lo, sorry about not to response to this forum but i have a hard trouble with my internet connection(a virus i guess), i wanna thank to the people who response to my question and to say that the information this ones forwarded me was really complete and useful. Thnx u all.

TXiceman
04-06-2007, 04:04 AM
There are some basic rules used in designing a defrost system, but you find, every engineer has his personal preference for defrost systems. If is part science and a lot of ART.

Ken