Hot gas defrost - What pressure?

[nh3curesall]

Hi everyone,
At work there is a debate on what is the correct back pressure setting for a hot gas defrost of a normal freezer. Some are saying 4 to 5 bar while others say 6 to 7 bar.
Anyone have there own thoughts on what pressure and their reason why?

[US Iceman]

Great subject. Instead of just giving you a pressure, let me give you something to think about.

What pressure do you need to hot gas defrost? Not the supply pressure..... the actual defrost pressure in the evaporator.

Now add up the pressure losses from the defrost regulating valve back to the compressor discharge. Add the two and you have the required defrost supply pressure. If you lower the pressure losses in trying to get the hot gas to the evaporator, you lower the required defrost supply pressure.

[Segei]

I agree that this is very interesting topic. Many people believe that it is simple question but it is not. Several questions. Temperature in the freezer, this is overfed or flooded evaporator. Is this pressure regulator located at outlet of the evaporator coil?

[US Iceman]

This is a lot easier to do if you are building a new system. Trying to fix these issues in an existing system takes a lot of work & $$'s.

[nh3curesall]

-20DegC room temp, flooded evap with a hanson reg valve in the suction line

[josef]

Hi, on a theme for many stories and stories, consider what each test is good for the system

[Peter_1]

Isn't t it better to speak about a diffrential pressure?

[RANGER1]

nh3curesall , all the settings are right as they are all above 0 deg C .
But in most cases + 10 deg C is most common ( 515 kpa ) .
Its best to have flow of liquid condensed out of defrost pressure regulator at some point earlier in defrost , so you know work is taking place by change of state .
You should always refer to temp instead of pressure as thats what we are trying to achieve in any part of frig cycle .

[LoveNH3]

More info:
Why is there such a diverse range of operating pressures for defrosting evaporator coils? Why do some use 95psig(6.5 bar) while others use 75psig(5 bar) if the gas is condensing above 32 degree F(0.0C)? Here is where we have some conflicts. The term hot gas implies the gas is hot. How hot does the gas have to be? If the primary heat source for defrosting evaporators is the gas condensing in the tubes, does temperature really control the defrost cycle? Yes, as long as the gas pressure for defrosting is sufficiently high to allow saturation temperatures above 32 degree F(0.0C). Consequently, providing higher temperatures is not required and, in fact, more expensive to operate.

The greatest quantity of heat energy is the gas condensing ( latent heat), not the actual gas supply temperature.

[Segei]

Important factor of BPR setting is temperature of surrounding air, because we have to defrost the tubes and the fins. Colder surrounding require additional heat to defrost the fins. So BPR setting should be 4-4.5bars for coolers, 5-5.5bars for regular freezer(like yours) and 5.5-6bars for extremely cold freezers. Hot gas supply should be adjusted accordingly.

[US Iceman]

More info:
Why is there such a diverse range of operating pressures for defrosting evaporator coils? Why do some use 95psig(6.5 bar) while others use 75psig(5 bar) if the gas is condensing above 32 degree F(0.0C)? Here is where we have some conflicts. The term hot gas implies the gas is hot. How hot does the gas have to be? If the primary heat source for defrosting evaporators is the gas condensing in the tubes, does temperature really control the defrost cycle? Yes, as long as the gas pressure for defrosting is sufficiently high to allow saturation temperatures above 32 degree F(0.0C). Consequently, providing higher temperatures is not required and, in fact, more expensive to operate.

The greatest quantity of heat energy is the gas condensing ( latent heat), not the actual gas supply temperature.

I made the same arguments in a recent edition of the RETA Breeze newsletter.

[US Iceman]

Important factor of BPR setting is temperature of surrounding air, because we have to defrost the tubes and the fins. Colder surrounding require additional heat to defrost the fins. So BPR setting should be 4-4.5bars for coolers, 5-5.5bars for regular freezer(like yours) and 5.5-6bars for extremely cold freezers. Hot gas supply should be adjusted accordingly.

I'm not sure if I agree with this. I think the requirement is for an increased ability to provide more hot gas to the evaporator as the coil temperature decreases. The temperature difference between hot gas saturation temperature for the gas pressure available and the coil temperature will mean gas condenses quicker with the increased TD. It would stand to reason that when this occurs the gas will condense faster, so to promote a reasonably rapid defrost the pipe supplying the hot gas should be larger in diameter.

If the pipe diameter is too small, then it would be a reasonable assumption that higher pressure is required to supply the mass flow necessary for defrost.

[sedgy]

because ,it hasent been mentioned I am just adding that too much hot gas for too long can stop comp winding cooling then problems result overheated comp,s

[LoveNH3]

In reply for US Iceman. I'm glad to see another member of RETA. That was quoted out of 2009 Issue #1. A lot of good information comes out of this organization.

[US Iceman]

In reply for US Iceman. I'm glad to see another member of RETA. That was quoted out of 2009 Issue #1. A lot of good information comes out of this organization.

I though that looked familiar. It's probably the short article I wrote on this subject.

[abet_meneses]

For a ordinary holding freezer on mechanical pump system BPR setting should be in range of 6 to 7.5 bar in a ordinary condition.we should also consider how far is the evap from discharge pipe.

[Segei]

I'm not sure if I agree with this. I think the requirement is for an increased ability to provide more hot gas to the evaporator as the coil temperature decreases. The temperature difference between hot gas saturation temperature for the gas pressure available and the coil temperature will mean gas condenses quicker with the increased TD. It would stand to reason that when this occurs the gas will condense faster, so to promote a reasonably rapid defrost the pipe supplying the hot gas should be larger in diameter.

If the pipe diameter is too small, then it would be a reasonable assumption that higher pressure is required to supply the mass flow necessary for defrost.

I think that hot gas supply should be balanced especially for flooded coils. Coil tubes can be defrosted very fast but it takes time to defrost the fins. Oversupply of hot gas create a lot of blow by gas and huge parasitic load especially for flooded coils. Overfed coils have small orifices which create internal flow restriction and they limit blow by gas. Flooded coils don't have this restriction and amount of blow by gas can huge.

[US Iceman]

I think that hot gas supply should be balanced especially for flooded coils. Coil tubes can be defrosted very fast but it takes time to defrost the fins. Oversupply of hot gas create a lot of blow by gas and huge parasitic load especially for flooded coils. Overfed coils have small orifices which create internal flow restriction and they limit blow by gas. Flooded coils don't have this restriction and amount of blow by gas can huge.

I agree with what you said here. I think this discussion gets into some areas where it would be helpful to be specific on my part. What you said is exactly correct for the current type of coil circuiting and defrost methods we use in this industry. The weak links in this (my opinion) are: the defrost relief valve (just a BPR) and the coil circuiting used.

The BPR is only sensitive to pressure, not condensed liquid which is what we are dealing with on hot gas defrost. If the coil pressure during defrost reaches the BPR setting it will open. It does not know the difference between gas or liquid. And as you have said bypassing gas is bad!

If we used drainers for removing the condensate then we do not need pressure regulators as long as the gas pressure stays slightly above the equivalent to 32°F (or 0°C). Liquid accumulates and the drainer removes it from the coil.

Coil circuiting is a whole other can or worms.

[Segei]

I agree with what you said here. I think this discussion gets into some areas where it would be helpful to be specific on my part. What you said is exactly correct for the current type of coil circuiting and defrost methods we use in this industry. The weak links in this (my opinion) are: the defrost relief valve (just a BPR) and the coil circuiting used.

The BPR is only sensitive to pressure, not condensed liquid which is what we are dealing with on hot gas defrost. If the coil pressure during defrost reaches the BPR setting it will open. It does not know the difference between gas or liquid. And as you have said bypassing gas is bad!

If we used drainers for removing the condensate then we do not need pressure regulators as long as the gas pressure stays slightly above the equivalent to 32°F (or 0°C). Liquid accumulates and the drainer removes it from the coil.

Coil circuiting is a whole other can or worms.

I think that every plant should have outlet pressure regulator for hot gas line. Hot gas supply to the coils with BPR can be adjusted to have blow by gas less than 5% of coil cooling capacity. To keep it at this level, pressure in hot gas line should be constant all year around. Oversupply of hat gas to the coils with liquid drainer creates a lot of condensate and it isn't easy to drain this condensate from the coils(especially from overfed coils). So hot gas line pressure regulator will be very useful for coils with liquid drainers as well.

[US Iceman]

...coils with liquid drainer creates a lot of condensate and it isn't easy to drain this condensate from the coils(especially from overfed coils).

Drainers do not create more condensate. That is a function of the coil defrosting and the hot gas supply.

I do agree though the overfed coils can be a problem, but this is a coil circuiting issue and how the headers are designed IMO.

And I do agree with with what you are saying about the regulator pressure. If the defrost supply pressure drops too low it does not matter whether you have drainers or BPR's. The coil will not defrost.

[Segei]

Drainers do not create more condensate. That is a function of the coil defrosting and the hot gas supply.

I do agree though the overfed coils can be a problem, but this is a coil circuiting issue and how the headers are designed IMO.

And I do agree with with what you are saying about the regulator pressure. If the defrost supply pressure drops too low it does not matter whether you have drainers or BPR's. The coil will not defrost.

Let's compare defrosting pressure in the coils(flooded). For the coil with BPR(set to 70psig) this pressure will be 70psig. For the coil with liquid drainer defrost pressure in the coil will be up to condensing pressure(120-150psig) if we don't have hot gas line pressure regulator. At 120psig we will have more condensate than at 70psig.

[MeanMack]

In my experience 120 - 130 PSI is typically enough to achieve an adequate defrost. Defrost time can also be something hard to figure into the equation if you are energy conscience. In theory when the defrost starts most of the HG will condense and return to the Engine room as liquid. In practice as the defrost is winding down there is a lot of hot gas returning to the compressor suction line. This can place a significant load on the compressors. A big step is reducing the number of defrosts. Too many of us defrost blindly with regimes set up decades ago. But on a zone that is defrosting check the coil temperature, when it reaches a predetermined temp (say 60 degF) you may look at stopping the defrost. To find the temp and length of defrost is not to difficult, just do some trial and errors. Keep lowering the temperature and time as far as you can and still get a clean coil.

[US Iceman]

In practice as the defrost is winding down there is a lot of hot gas returning to the compressor suction line. This can place a significant load on the compressors.

And that is the problem I see with using pressure regulators to control defrost.

[Segei]

In my experience 120 - 130 PSI is typically enough to achieve an adequate defrost. Defrost time can also be something hard to figure into the equation if you are energy conscience. In theory when the defrost starts most of the HG will condense and return to the Engine room as liquid. In practice as the defrost is winding down there is a lot of hot gas returning to the compressor suction line. This can place a significant load on the compressors. A big step is reducing the number of defrosts. Too many of us defrost blindly with regimes set up decades ago. But on a zone that is defrosting check the coil temperature, when it reaches a predetermined temp (say 60 degF) you may look at stopping the defrost. To find the temp and length of defrost is not to difficult, just do some trial and errors. Keep lowering the temperature and time as far as you can and still get a clean coil.

In my experience hot gas defrost can be done at 100psig or lower head pressure. Oversupply of hot gas is one of the reason of poor defrosting.
1. A lot of blow by gas will go to the suction.
2. This oversupply create a lot of condensate and it isn't easy to drain this condensate from the coils.

[MeanMack]

In my experience hot gas defrost can be done at 100psig or lower head pressure. Oversupply of hot gas is one of the reason of poor defrosting.
1. A lot of blow by gas will go to the suction.
2. This oversupply create a lot of condensate and it isn't easy to drain this condensate from the coils.

Great! Do you have any blast cells or freeze tunnels or any high humidity product like beef? How long do your defrosts typically last? What operating head pressure do you typically run? I like to see wetbulb plus a approach of between 12-14. In the winter months that can usually mean 90 psi.

[Segei]

Great! Do you have any blast cells or freeze tunnels or any high humidity product like beef? How long do your defrosts typically last? What operating head pressure do you typically run? I like to see wetbulb plus a approach of between 12-14. In the winter months that can usually mean 90 psi.

1. Average defrost lasts 30min., but it depends of temperature in cold room, size of the coil, type of the coil...
2. The lowest head pressure can be as low as 60-70psig.
3. Where is so warm winter? 90psig is 59F. Winter wet bulb temp. 45-47F. Dry bulb will be around 50F. I like this place. In Canada winter dry bulb very often can be below 0F.

[US Iceman]

The lowest head pressure can be as low as 60-70psig.

Yeah, I would agree with that. If the coils are designed to drain condensate properly and you can get enough hot gas to them it should work.

[Segei]

Many people focused on minors energy savings. One cold storage chief engineer decided to shut the light off during the breaks and lunches. It worked for a while until one guy was left in the dark. I estimated the energy savings and they were $2 per day. One coffee for one person. Not to much. Optimization of head pressure is major energy savings opportunity. Very often it can save up to 50% of total energy savings.

[US Iceman]

Optimization of head pressure is major energy savings opportunity. Very often it can save up to 50% of total energy savings.

Not to mention what you can do for demand reduction!

Shutting off lights for a few sporadic times probably wastes more energy than it saves.

I think people work on energy savings in industrial systems the same way they do in commercial refrigeration. It usually devolves into something like this... What type of gadget can we install to save energy?

The energy savings are in the optimization of the system itself.

[Segei]

Not to mention what you can do for demand reduction!

Shutting off lights for a few sporadic times probably wastes more energy than it saves.

I think people work on energy savings in industrial systems the same way they do in commercial refrigeration. It usually devolves into something like this... What type of gadget can we install to save energy?

The energy savings are in the optimization of the system itself.

Demand reduction is very interesting and complicated topic. Some people shut the plant off when energy is expensive. Later they run it at full capacity. Structure of energy contract is very important issue for this operation.

[MeanMack]

In my experience hot gas defrost can be done at 100psig or lower head pressure. Oversupply of hot gas is one of the reason of poor defrosting.
1. A lot of blow by gas will go to the suction.
2. This oversupply create a lot of condensate and it isn't easy to drain this condensate from the coils.

That is why you monitor the temperature on the suction line of the evaporator and stop the defrost after that temp is reached with a little time delay and greatly minimize the amount of HG returning to the compressors suction.

[Segei]

That is why you monitor the temperature on the suction line of the evaporator and stop the defrost after that temp is reached with a little time delay and greatly minimize the amount of HG returning to the compressors suction.

Evaporator pipes will defrost fast. It takes time to defrost the fins. This is time when a lot of blow by gas will go to the suction. It is the reason that many companies defrost one coil at time. Even at this approach load will increased. I have done test for one plant. Total are 16 coils. 8 coils were defrosted simultaneously with slight increase in suction pressure. I estimated that balanced supply of hot gas can give 5-10% of blow by gas compare to cooling mode operation. Over supply can increase this numbers up to 50%. This is information about the coils with back pressure regulators. Liquid floats can give us 1-2%.

[sterl]

Years ago: Lane Lloyko made a presentation about using Float Drainers on cold storage type coils....His study indicated that a flow Velocity was required to make condensate move in the evaporator tubes and this was identified for the specific tubes/fin arrangement and so on for those specific coils. This for a multitute of models and manufacturers of coils....

The resulting "overfeed" of hot gas was a small fraction of that which a regulator would pass after the "melt" was complete; but did depend somewhat on the coil circuiting and tube diameters.

I found this in the IIAR archives from 1982 presentations....

[US Iceman]

A lot of the issues we have with hot gas defrost are built into the system. We have evaporators with button orifices for liquid distribution. These can interfere with the condensate draining during defrost. During a defrost cycle if you can't get the liquid out of the coil it will not defrost properly.

An evaporator in defrost should be just like a steam coil during heating. You put hot vapor into the coil, it condenses and warms the coil (and then the air) and the steam trap meters the liquid at the rate the vapor condenses.

Defrost is just like a cold start on a steam coil. Because the coil is cold it will condense a lot of steam, hence the trap has to be large enough to handle the greater volume of condensate. This safety factor for sizing the traps is about 3-4 times the hot operating condition for condensate formation. Therefore, if the trap does not have sufficient flow capacity the liquid builds up in the coil and the heating effect is reduced (since it has started to become liquid locked).

If you can supply a sufficient volume of hot gas to a coil and drain the liquid as fast as it forms the coil should defrost very quickly.

We keep trying to re-invent hot gas defrost and the steam guys have been doing properly for many years.

[smpsmp45]

I am not sure but it was an article by Parker Valves. it also has do with the Sp heat at that pressure. At lower pressures that Sp heat being more Parker recommends @ 100 PSI pr. ( I could be wrong n the figures.) I shall check if I can get that data