Hot gas defrost

[spr2003]

I apologize with the moderators, may be somebody already asked that, but couldn't find any thread with the search function.
Has anyone experienced hot gas defrost system on industrial or hipermarket low temperature plants?
I just did it with a scheme wich allows the gas pushed into the evaporators to condense itself defrosting the coils and after that it goes in the liquid pipe, ready to feed the other evaporators. Two no return valves and three solenoid valves any evaporator makes the system a bit complicated. To get the right pressure drop to allow the hot gas to warm the coils, two differential pressure valves are provided on the discharge pipe of the rack and on the liquid pipe.

Any suggestion?

Thanks
Sergio

[mystry]

your design is good to run with defrost and cooling at the same time and this pressure regulator valves provide the defrost faster but i think it is possible to use suction manifold such as differential valves to provide pressure drop on the discharge line. İ searched any literature but all of the systems use these differantial valves.

eg

[Sanel]

Hot defrost is definitively one of the ways. My opinion is that this cind of defrost is not economical. Usually the good way of using this heat is for sanitation water or warming.

Sanel

[US Iceman]

Hot gas defrost is the main type of defrost method used in industrial refrigeration systems. In industrial systems the piping arrangments and valves used are almost always different than the hot gas defrost system used in commercial refrigeration systems.

In an industrial system you will normally see three or four pipes used.

Suction
Liquid
Hot gas supplyor,

Suction
Liquid
Hot gas supply
Condensate returnIn commercial refrigeration systems you typically see a different piping configuration where the condensate is pushed back into the liquid lines.

One of the main problems with hot gas defrost systems is the regular expansion and contraction of the pipes as they warm up and cool down. This adds additional stresses to the piping and can lead to premature failure/leaks.

Hot gas defrost is a very good system since the evaporator is defrosted from the inside, instead of from the outside (like electric defrost).

Economics depends on many things. Hot gas defrost piping can be more complicated and expensive to install.

Electric defrost is easier to install. The electrician does the wiring, so his cost is not in the refrigeration contractors costs.

On the other hand, the additional piping of the hot gas defrost system is included in the refrigeration contractors costs.

Several of the other moderators may have some comments to add to this also.

[Josip]

Hi, all :)


Hot defrost is definitively one of the ways. My opinion is that this cind of defrost is not economical. Usually the good way of using this heat is for sanitation water or warming.
Sanel

I do not believe there is any economical way of defrosting;) We have to spend additional energy to remove ice from evaporators to reduce the cost of cooling. This step in cooling process is unfortunately inevitable:(


Hot gas defrost is the main type of defrost method used in industrial refrigeration systems. In industrial systems the piping arrangments and valves used are almost always different than the hot gas defrost system used in commercial refrigeration systems.

In an industrial system you will normally see three or four pipes used.

1. Suction
2. Liquid
3. Hot gas supply

or,

1. Suction
2. Liquid
3. Hot gas supply
4. Condensate return

In commercial refrigeration systems you typically see a different piping configuration where the condensate is pushed back into the liquid lines.

One of the main problems with hot gas defrost systems is the regular expansion and contraction of the pipes as they warm up and cool down. This adds additional stresses to the piping and can lead to premature failure/leaks.

Hot gas defrost is a very good system since the evaporator is defrosted from the inside, instead of from the outside (like electric defrost).

Economics depends on many things. Hot gas defrost piping can be more complicated and expensive to install.

Electric defrost is easier to install. The electrician does the wiring, so his cost is not in the refrigeration contractors costs.

On the other hand, the additional piping of the hot gas defrost system is included in the refrigeration contractors costs.

Several of the other moderators may have some comments to add to this also.

There is not too much to add to your points;) maybe to use water with hot gas or only water and then reuse that cold water on condensers and then again reuse warm water from condensers for defrosting.

Here I am thinking about big industrial plants having available hot gas for defrosting all the time. For pocket plants it is Ok to use electrical heaters for defrosting.

The biggest role in defrosting design play money:D

Best regards, Josip :)

[NH3LVR]

:) I have had a little experience with Hot Gas mixed with water. In one application we froze Salmon in batches (Blast Freezer, NH3, Flooded) We used a Dry Sumped Condenser and pumped water from the tank at the same time we turned on the Hot Gas. It was fast and the incomplete defrost often seen in Blast Freezers was absent.

Another application was a Containerized Blast Freezer (13B1, DX,). The original configuration used only Hot Gas. The Hot Gas was supplied by a False Load Coil in the Condenser which was fed by a TX Valve. It did not work well. The Liquid Level in the Receiver would soon run low (Ending up in the Suction Accumulator), leaving you no choice but to pump out the coil and start over. This let you gain a bit of ground, but hardened what remained. It was a poor system and required going in with a hose and defrosting the entire freezer on a regular basis.

My solution was found in a old 110 Gallon water heater which was lying around. I modified the Thermostats to produce 70F (24C) water, built a manifold, and installed hand valves. (This does require thought to the piping and draining of the lines.) We went from incomplete defrosts that took 40 minutes to a excellent defrost in 8 minutes. The only down side was that if you left the water on too long the 2 pan drains would have overflow, as they were not sized for Water Defrost. I stopped by the plant when I was working in Alaska last Summer and it is still in use. (I converted it to R-22)
The combination of water and Hot Gas is viable and effective, but must be installed correctly.

[US Iceman]

Water defrosting can indeed be a little tricky. If the water is too warm you get steaming inside of the room. If the water is too cold the defrost takes too long.

The only time I have seen water & hot gas is on production freezers (tunnels) where they want the turn around time as short as possible.

I also believe we can do better for hot gas defrosts. If you can get sufficient hot gas to the coil during defrost, and able to drain away the condensate as fast as it forms you should be able to hot gas defrost a coil in under 10 minutes.

I think the normal time we see of 30 minutes or more (for the defrost duration) is due to how we pipe and control hot gas defrosts.

[DeB]

I have seen good results on commercial systems with reverse cycle. A bit more complicated than Hot gas but less problems withn flood back.

[spr2003]

I ran the plant since a couple of weeks, and now i'm in the fine tuning.

It works really great defrosting in these conditions:
-def stop temp: +18°C
-drip off time: 6 min.
- fan start temp: -15°C

This is a supermarket plant with around 40 low temperature counters ARNEG brand and electronic on-off expansion valves.

[johnyfreon]

I'm mostly familiar with this type

http://img102.imageshack.us/img102/9374/superev3.png

[The MG Pony]

^ Blurry! (The message you have typed is too short)

[johnyfreon]

^ Blurry! (The message you have typed is too short)


“Ezekiel 25:17”

“The path of the righteous man is beset on all sides
By the inequities of the selfish and the tyranny of evil men.
Blessed is he who, in the name of charity and good will,
Shepherds the weak through the valley of darkness,
For he is truly his brother’s keeper and the finder of lost children.

And I will strike down upon thee with great vengeance and furious anger
Those who attempt to poison and destroy my brothers.
And you will know my name is the Lord when I lay my vengeance upon you.”

[The MG Pony]

“Ezekiel 25:17”

“The path of the righteous man is beset on all sides
By the inequities of the selfish and the tyranny of evil men.
Blessed is he who, in the name of charity and good will,
Shepherds the weak through the valley of darkness,
For he is truly his brother’s keeper and the finder of lost children.

And I will strike down upon thee with great vengeance and furious anger
Those who attempt to poison and destroy my brothers.
And you will know my name is the Lord when I lay my vengeance upon you.”


Tsk tsk, the forum is no place for mythology!

I was refering to what the forum said about my post, that it was too short, I hate them things.

[karelsdr]

To help you play the hand your dealt.First one of those regulating valves must be an interupter valve. For example your discharge pressure must rise above your liquid pressure during the defrost cycle.Keep your liquid pressure consistant so your liquid injection valves stay reliable. Raise your head with that interupter during any defrosting then allow it to equalize during normal operation to maximize energy savings

[spr2003]

I'm mostly familiar with this type

http://img102.imageshack.us/img102/9374/superev3.png


Yes, that's the scheme of the plant we build.