Hot gas defrost system

[tadipatrir]

I am new to this forum.

My problem is I want to design hot gas defrost system for cold room by replacing existing electrical heater defrost. Now my idea is to follow this sequence. a) Pump down of the system before defrost. b) Bypass the discharge gas to the liquid line after receiver. c) All the gas to pass through TXV and through evaporator to melt the ice.

What are the possible problems in this concept.

[lana]

Hi tadipatrir,

In hot gas defrost you have to pass some of the discharge gas to the evaporator. You can not pump down and close the liquid line.
There must be a solenoid valve in the hot gas line which is opened during defrost cycle.
Also you must install an accumulator before the compressor suction.
Good Luck
Cheers

[tadipatrir]

Hi Iana,

Thanks for your reply. I do have an accumulator installed in the suction line. Also I want to sned the remaining discharge gas after pump down through TXV and evaporator so that in case of condensation and subcooling in the evaporator not much liquid will be generated. Ofcourse all this with evap. fan in off position. Also this will be a temperature terminated defrost. My only doubt is how TXV will behave under this circumstance as to how it will react to the gas and superheat feedback it gets from the bulb after evaporator.

Cheers

Raghu

[lana]

Also I want to sned the remaining discharge gas after pump down through TXV and evaporator so that in case of condensation and subcooling in the evaporator not much liquid will be generated.

When the system is pumped down then there will be no "remaining gas". Everything is in the condenser and the receiver.

My only doubt is how TXV will behave under this circumstance as to how it will react to the gas and superheat feedback it gets from the bulb after evaporator.

You direct the hot gas after the TEV and before the distributor. The other portion of the liquid which is coming from the receiver goes into the TEV as normal cycle.
Of course this is until there is no frost left. When frost is gone and hot gas defrost is continued then there will be problems, like compressor overload.
I had this in an ice machine. There was a problem with the ice breaking from the mold and droping into the bin, then there was no signal for defrost termination. Hot gas defrost continued without any energy taken from it. In this case I had compressor overload.

Cheers

[lana]

If you are interested PM me and I will send you a file about defrost.
But don't tell anybody that you get it from me.

Cheers

[tadipatrir]

Dear Iana,

Please sne me that paper on defrost. I promice that I will not tell any one that I got it from you.

Cheers!

[lana]

Dear Iana,

Please sne me that paper on defrost. I promice that I will not tell any one that I got it from you.

Cheers!

It is LANA.
Send me a PM with your e-mail address. Don't put your mail here on the posts.
Cheers

[tadipatrir]

Dear Iana,

What is PM? Can you clarify.

Cheers

Raghu

[lana]

PM = Private Message

Look at the top right corner of the screen, click the "Private Messages" and then send one to me with your mail address.

[Samarjit Sen]

Hi Lana,

For my Cold Rooms, I used to apply hot gas system. I had a separate Solenoid valve for the hot gas line and Crankcase Pressure regtulator just on the suction line before the compressor. It was very effective, but the compressor kept on running. Then I started using electric defrost which I found a better option as the compressors got rested during this period.

I would be too happy if you could tell me a better way to defrost the coils in Cold Rooms.

Thank you

[lana]

There are many ways to defrost a coil.

1- Electric
2- Hot gas
3- Air circulation
4- Water
5- Reverse cycle

Honestly I prefer electric one, because there are almost nothing to worry about.
But the other methods work well too, considering the application.
For cold room application (-20°C) the electric defrost works great. In blast freezers I have seen hot gas and water defrost also, which works as well.

This is one of those areas which there are different opinions. There is no absolute solution I think.

Cheers

[tadipatrir]

Lana,

I had put a PM to you. Please go through and respond.

[Samarjit Sen]

We had used Hot Gas for all our Cold Rooms. Now we are using Electric Defrost system as it has no problems. We are using Timers to activate the Defrosting system. Instead of this I wanted to know if there are other ways such as activation on reduction of air flow or formation of ice over the coil tubes.

[lana]

Lana,

I had put a PM to you. Please go through and respond.

I did.
Cheers

[lana]

Instead of this I wanted to know if there are other ways such as activation on reduction of air flow or formation of ice over the coil tubes.

I have read and heard that there are sensors for defrost activation but I have not used them as they are not available here.

• frost thickness
• air pressure drop

Cheers

[Samarjit Sen]

Can any one give me a reference of the sensors for the Defrost of the Coils.

[US Iceman]

What you are looking for is a difficult concept to put into practice known as "demand defrost".

Some time ago Alco made a system that used photo-electric eyes positioned in such a way to measure the frost thickness on the air entering side of the coils. Apparently, they were not very succesful or we would be seeing more of this.

Others have tried to use sensitive static pressure measuring devices to monitor the static pressure loss over a coil as it frosted. Again, I have not seen this used very much.

In both examples, if these were such valuable ideas and practical in application these would be very common.

A better concept is to use improved hot gas defrost systems that defrost faster with less negative impact on system performance.

[US Iceman]

Here is something that might help on this subject...

http://www.parker.com/rs/90-11a.pdf (about 1.5 MB)

[TXiceman]

As Iceman noted, there have been various attempts at demand defrost to only defrost when needed. Each has had varying degrees of success incertain applications.

The problem is to find a reliable and cost effective method of measuring the amount of frost or ice on the coil.

I still prefer the old reliable time initiate and terminate method. It does take some degree of monitoring to makes ure it is working, but it is more reliable than the demand systems.

Ken