Hot Gas Defrost

[Abe]

Installing a new system, just realised evaporator does not have electric heaters. its a 20 Kw evap

So , Im going to have to go hot gas defrost

The check valve.........does it go after the expansion valve

Its better I look like an idiot asking, but I want to get it right

Thx

[vaidas_78]

The chek valve should be installed parallel to TEV. The chek valve direction-to liquid line.

Vaidas

[vaidas_78]

Ignore that I wrote above if your system has one compressor and evaporator. In that case you don't need the chek valve, if the liquid line has a solenoid of course.

[chillyhamster]

If your system is a single evporator and unit, then you will not be able to use a two pipe hot or sat gas defrost method and will have to use a three pipe system. Take a 1/2 or 5/8 tee off the discharge and pipe (or top of reciever, if it contains enough volume of sat gas vapour to effect defrost) to a tee between the expansion device outlet and the distributor with a solenoid valve for control. The suction gas pressure to the compressor should be controlled with a crankcase pressure regulator. In practice it is a pretty crap way (Comp nearly always runs 100% of the time and eventually fails) of effecting a defrost and you would be far better to fit defrost heaters.

[wambat]

Take a look here http://www.sporlan.com/5-158.pdf:)

[frank]

Hi Wambat.

The link doesn't work for me

[botrous]

I think that the link should be without the
like this i think it will work http://www.sporlan.com/5-158.pdf , nothing should be after the final extension of a file , that's what's i've mentioned from my experience in computers

[Dave Goodings]

Hello Aiyub

The post the guys put up for you from Sporlan is NOT for defrost purposes this is for low load conditions/capacity control on a/c systems you would need more controls i.e. suction accumulator/pan loop/ solenoids/etc for defrost use

regards
Dave G

[wambat]

Dave is correct, I believe this is better http://www.bmil.com/ballyrefrig/thermosaver.htm

[Peter_1]

Hi mate,

Is this a freezer application?
My opinion, I should install heaters, vene if you have to remove the evaporator once again.
Like the posters said, if it's a 1 evap/1 cond situation, you will also need to to install a TEV on your condensor and 2 check valves. A lot of work and the material will cost more than the heaters.
In case of a reversal cycle, you will also need a large enough liquid separator.
Otherwise your compressor will pump liquid after a defrost.

[vaidas_78]

Hi mate,

Is this a freezer application?
My opinion, I should install heaters, vene if you have to remove the evaporator once again.
Like the posters said, if it's a 1 evap/1 cond situation, you will also need to to install a TEV on your condensor and 2 check valves. A lot of work and the material will cost more than the heaters.
In case of a reversal cycle, you will also need a large enough liquid separator.
Otherwise your compressor will pump liquid after a defrost.

Hi Peter, what do you mean "you will also need to to install a TEV on your condensor " and "In case of a reversal cycle, you will also need a large enough liquid separator"?

[Abe]

Hi mate,

Is this a freezer application?
My opinion, I should install heaters, vene if you have to remove the evaporator once again.
Like the posters said, if it's a 1 evap/1 cond situation, you will also need to to install a TEV on your condensor and 2 check valves. A lot of work and the material will cost more than the heaters.
In case of a reversal cycle, you will also need a large enough liquid separator.
Otherwise your compressor will pump liquid after a defrost.

Peter.

Yes, it is a freezer room.

Ive decided to install heaters, instead of going the hot gas defrost route

I ordered 8 straight heater coils, cost £20.00 each. Total cost £160.00

Fitted them today, now I dont have to worry about check valves and compressor burning out, etc

Thanks for everyones help........I appreciate it.

Abe

[Peter_1]

Hi Peter, what do you mean "you will also need to to install a TEV on your condensor " and "In case of a reversal cycle, you will also need a large enough liquid separator"?

http://www.bmil.com/ballyrefrig/thermosaver.htm is one example

[vaidas_78]

http://www.bmil.com/ballyrefrig/thermosaver.htm is one example

Sorry Peter, but I still can't see TEV on the condenser.All I can see is a 3-way solenoid valve.

[Peter_1]

You're right, this wasn't a good example.
This is a better one http://www.books4hvac.com/rb/Heat_Pumps1.htm
I hadn't look very weel when I posted this link.
They use a 3 way valve instead of a 4 way

BTW, what do you guys think of the Thermosaver system?
Two liquid separators, one with a succool coil in it.
Why 2?
Is the subcool coil in defrost mode acting as a small condensor?
Where is the heat coming from to defrost the coil? Only from the absorbed power of the compressor?

Where happens the expansion?
What about excessive compressor SH?

For smaller applications like small ice machines and small evaporators this will work fine but for the system like Aiyub, I doubt.
Any comments on this?

[vaidas_78]

Hi Peter, it looks to me you have been confused.Thermosaver is not a heatpump and it use a 3way valve for defrosting, not for a cycle reversing.In this system the hot gas and a liquid are sharing the same pipe,so it needs 3way valve and two chek valves.
The subcool coil in the defrost mode forces the liquid to boil in accumulator.
The crankcase pressure regulator(pos.H) protects compressor from overloading during the defrost cycle and also evaporizes some liquid.

Where is the heat coming from to defrost the coil?

The vaporized refrigerant picks up some heat passing through the suction line. As it passes through the compressor, the heat of compression raises the temperature to such a level that it continues to heat the evaporator and remove the frost.

Regards Vaidas

[Peter_1]

.....,so it needs 3way valve and two chek valves.
The subcool coil in the defrost mode forces the liquid to boil in accumulator.
The crankcase pressure regulator(pos.H) protects compressor from overloading during the defrost cycle and also evaporizes some liquid.

I hadn't noticed valve H, thought it was a shut-off valve.
Valve H act while defrosting as an AEV.

The vaporized refrigerant picks up some heat passing through the suction line. As it passes through the compressor, the heat of compression raises the temperature to such a level that it continues to heat the evaporator and remove the frost.

I do understand the schematic, but you're confirming what I'm saying -I think- thermodynamically seen, the only heat we're adding to the system is the absorbed power of the compressor on that moment, which will very litlle due to the very small DP the compressor is working.
Further more, the liquid coming from the evaporator encounters a serious temperature drop when entering the right accumulator (with the subcool coil)
So the hot discharge gasses going to the evaporator for defrosting will undergo a serious cooling, so the net availbale heating effect will seriously decrease.

[vaidas_78]

Hi Peter,
The main parameter for the discharge temperature(and also pressure) increasing is a compression ratio. Higher compression ratio - higher the temperature at the end of the compression.Low temperature compressors have the high compression ratio.What's why they cannot be used in high temperature aplications as a high suction pressure would increase the discharge temperature and pressure greatly.For this reason thermosaver is using the crankcase pressure regulator. During a defrost cycle DP is not very high but it's normal .Of course the suction gas temperature affects the discharge temperature but just slightly.
What I don't like in this system is that the hot gas shares the same pipe with the liquid.At the beginnig of the defrost the hot gas will push the liquid all along the line and evaporator to the suction line.What's why they are using two liquid separators,I thik. What would happen if the liquid line would be very long?
I would rather choose a three pipe system whith the one accumulator.
What do you think?