Hot Gas vs. Electric Defrost

I was all excited about fixin' up my customers meat warehouse when his 10 HP condenser went out. The original box design is 35degF, but now we're going to design at 28degf. AS such, the existing system employees air defrost unit coolers.

Since there is no existing electric heat circuit at the unit cooler locations, I was dreaming of three 7.5 HP R-404A systems @50,000 BTUH ea. utilizing hot gas defrost. My reasoning was fewer electrical components to have to service while standing on the lift 25 feet in the air, and simplifying the installation.

After confirming my load calculations with Bohn engineering, I had already decided on BHG450 unit coolers, and asked "Glen" if he'd recommend C.U.'s. He first stated that Bohn only incorporates Hot Gas in their racks. But he justified utilizing electric defrost instead by stating that that the ELECTRIC is less maintenance, irregular defrost patterns of hot gas, and that electric heater service typically does not exceed min. circuit ampacity of the C.U. (so that by utilizing defrost relay lockout switches, there is no consideration for higher rated or additional electrical service for the heaters).

What do you think? Is Glen just selling the BDT condensers, or does that seem reasonable to you?

I don't think Glen is being insincere. The merits of electric versus gas defrost are subject to ongoing debate. Electric defrost will require power for the heaters and a termination stat run between the condensing units and coils they operate. There are several ways to wire it all up, but you will need some new wire for electric defrost. He may be right that the power to the units does not have to be upgraded.

If you had parallel compressors a gas defrost would make sense, but it makes less sense for conventional units. A heat pump setup would make sense but only for indoor equipment in your area.

After thinking, I tend to agree with Glen for your situation.

Dan

Thanks, Dan.

I have no experience in the parrallel thingy. A very brief explanation would be helpful to me, but I guess I'll read up in the Modern Refrigeration text.

As far as the wiring; I do that well, as I sleep. Of course the Bohn BHE that unit cooler that will be employed will be equipped with defrost terminat/fan delay..... no problem. Also the C.U. will be factory equipped heater relay (I better confirm that).

Later......;)

Well, simply put, a gas defrost has the advantage that it is "free." No new expenditure of energy. We find this advantage on parallel systems because we are switching the heat removed from other things refrigerating to something that needs a defrosting. Parallel systems have this advantage. Some compressors are removing heat and we can employ a switching valve to send all that heat to a system requiring heating.

Heat pumps are similar in that they utilize what heat they can gather from what we normally consider cold air and use it to advantage to heat an air space.

Conventional equipment can utilize a gas defrost to advantage by storing heat during the refrigeration process... hmmm. Kramer-Trenton's Thermo-bank comes to mind.

I have also seen units that employ hot gas which basically accomplish nothing besides adding the motor heat to the refrigerant. It works well for small deli supermarket freezers.

Gas defrosts are shorter than electric defrosts. Gas defrosts that utilize an outside heat load have some energy advantages, but if you do a calculation you might think that it is not such a great advantage, especially in your situation, where the kw would not amount to much with electric defrost.

The above advice comes from a former promoter and supporter of the advantages of gas defrost versus electric defrost.

It is not simply a woman's prerogative to change one's mind.:)

I think I understand, now. In the case of parrallel systems, the hot gas defrost actually serves to compliment the condenser if the defrost program is timed properly, of course in conjunction with the application of a flooded condenser control.

Am I right?

Yep. Using heat that you collect from places that you don't want it to places that you can use it.

Dan

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Originally posted by herefishy


My reasoning was fewer electrical components to have to service while standing on the lift 25 feet in the air, and simplifying the installation.

But he justified utilizing electric defrost instead by stating that that the ELECTRIC is less maintenance, irregular defrost patterns of hot gas,

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Having both in a coldstore, hot gas is by far the easiest to look after and you are not hanging off a wobbly platform in order to do the work. :D

Irregular heating of coils with hot gas defrost can be cured with some cheap and intelligent mods and fixes once you sus out where the cold spots are.

I increased cycle times from 30 minutes to 2 x 45 minute periods per day and insulated the casing of the coolers with 50mm thick polystyrene to reduce heat draining away and to keep it where it was needed.

The bigggest problem was a human one - telling the drivers to shut the #*!@"! doors to the store.

The only other possible snag with hot gas defrost is the pipework subjected to the freeze \ thaw action. If the brazed joints aren't right, they have been known to split. If you use steel, they corrode fairly quickly if not adequately protected.

Steve

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Having both in a coldstore, hot gas is by far the easiest to look
after and you are not hanging off a wobbly platform in order to
do the work.

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There you have it. The age old argument. I once did a study on the expense of electric versus gas defrost with energy considerations and was caught in the debate regarding the higher cost of maintenance of gas versus electric. I have since noted that in the colder climates, electric defrost is more popular. The advantage of lower head pressures apparently supersedes the advantage of gas defrost for more than a single reason:

1. If you are letting head pressures float, gas defrost becomes more troublesome.

2. Gas defrost requires more controls and confuses the average mechanic, who now
has to think holistically.

Personally, I favor gas defrost, but have been whipped by the best who think otherwise.

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Originally posted by Dan


1. If you are letting head pressures float, gas defrost becomes more troublesome.

2. Gas defrost requires more controls and confuses the average mechanic, who now
has to think holistically.

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We use evaporative condensers, set between 10 - 12 bar, so pressures do not float that much

More controls? We use (1) Danfoss PM pressure contol valve with CVP pilot and (2)Danfoss solenoid valve and a multicore cable from the main control room. If it goes wrong, I can work in the warm bit.

The electrics need thermostats, switches, current and earth protection, distribution boards, cables, jointing boxes, and someone daft enough to work in temperatures of -24C to sort them out when it goes wrong.

As regards confusing the average mechanic, some of the horrors I have seen on electric systems seem to make gas systems a very attractive proposition :mad:

With electric, you can't see it or smell it, but if it goes badly wrong, you can definatly feel it
:eek:

Steve

Well, I'm glad that other techs appreciate my consideration regarding the service of the electrical defrost thing at 25' in the air on a wobbly platform!!! That somewhat makes me feel less ignorant!

However, I do appreciate the consideration of compressor heating......, that the wear on the "cooling" component of the system (compressor) being utilized to perform an opposite function, without any benfit to the cooling process itself. Furthermore, what is more efficient in terms of wattage?... the inductive circuit of the compressor motor, or the resistive circuit of an electric heater?

I find that the (adding heat to compressor motor) argument very valid.

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[i]. Furthermore, what is more efficient in terms of wattage?... the inductive circuit of the compressor motor, or the resistive circuit of an electric heater?

[/B]

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I look at it this way. The compressors have to run to cool the store anyway, so any work done to defrost a cooler, say 20kw, is diverted away from the job of cooling. No more compressors need to run to do the defrost, it is just that some of the capacity is bled away.

Electric heaters sit there for 22 hours of the day doing nothing, and then require 20kw of dedicated power to do the job and that goes directly on the electric bill. For the 22 hours they sit there, the cost of the installation isn't being used, it just sits there and we can't use it to do anything useful like make the tea or provide hot water for the washbasins.

Who is going to sit down and effectively divide up the load factors between cooling and hot gas defrost to justify the assertion one way or the other? As the motor and compressor have been running anyway, and "warmed up ready to go", it strikes me the gas method is more efficient.

Reactive or inductive? Don't know, better ask an electrician. I'm just a hairy assed ammonia sniffer who takes pleasure in watching other peoples discomfort when some of it leaks out :D

LOL!!!!!!!!!!!!!!......

Steve, you are very convincing, I must say.....

LOL, Steve. But we are blurring the issue. A gas defrost with a simplex, conventional, unitary unit is quite different than a gas defrost with parallel compressors. If I understand the original question, we were dealing with dedicated compressor units, which, of course are not normally running during a defrost. If you outfitted them with heat pump valving, maybe you could gain advantage, but in cold weather heat pumps can hardly manage to cool a room much less provide a hot gas defrost.

When it comes to wattage, watts don't really care whether they are born of resistance or induction. Watts are a pure measure of work and heat. If you want to calculate how much heat a self-contained freezer will put into your garage, all you have to do is look at the wattage.

You don't have to splice out the condenser fans or whether the door was left open, or whether it is hermetic or semihermetic.... efficient or inefficient. Whatever happens becomes predictable and measurable heat no matter how you get there... watt-wise, that is:)

Now if we are still attempting to make a case for a parallel unit.... Expect the price to double.

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Originally posted by Dan
If I understand the original question, we were dealing with dedicated compressor units, which, of course are not normally running during a defrost

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For clarity, that is the issue at hand.:)

The general concensus of opinion in the electrical department is that inductive is a bit more efficient than resistive, and they don't like hanging off wobbly platforms as well - so that opinion may be biased ;)

Steve

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The general concensus of opinion in the electrical department is
that inductive is a bit more efficient than resistive, and they
don't like hanging off wobbly platforms as well - so that opinion
may be biased

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The Wobbly platform part I certainly understand. But I disagree that there is such a thing as "inductive" and "resistive" wattage. Whatever goes in must come out.

De-icing a coil requires wobbbly platforms, no matter what sort of defrosting method.

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Who is going to sit down and effectively divide up the load
factors between cooling and hot gas defrost to justify the
assertion one way or the other?

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Actually many people have and do still. Without getting into the equations, here is the line of thought:

Gas defrost efficiency falls off dramatically once you lower discharge pressure. If it is a parallel rack, you have several compressors operating at higher pressures than they would otherwise have to operate at, simply to provide a defrost for one system that may be a small part of the total load, and then again for another.

Electric defrost begets a simplicity. Gas defrost begets complexity. I am playing the devil, but there are good arguments both from energy and maintenance expense for electric defrost.

Dan

Okay,

My take on if it is a single unit...use electric defrost. You make life really hard on that compressor by forcing it to run during a defrost with no load to pull down.

But....if you have a larger system (ie, more compressors), I feel hot gas is the ticket baby. Hot gas defrost will defrost the coil faster and more completely if set up correctly. Also, if you have quality workmanship, you will not have problems with breaking brazed joints.

It was mentioned about floating the head pressure would be troublesome for hot gas defrost. How is this so? I normally design hot gas defrost systems with a differential discharge regulator. This regulator senses the difference between hot gas pressure feeding to the coil and liquid pressure. This way, it always mantains a set pressure differential to "feed" hot gas to a coil.

I see that I have been away way too long. There are a lot of great discussions going on here and I would love to get involved again. Work gets busy and hectic, so talking about what I do for a living sometimes gets to me, but the topics you guys discuss just gets me interested. Keep up the good work!
Norty

I deal with hot gas defrost on a day to day basis, and belive me my preference is gas defrost. Have you considered a small parallel rack using hot gas defrost. You can use a three pipe hot gas loop system. I have currently been doing a lot of installations of this type and find it to be quite simple. Your piping is at a minimum and with a few valves your installation is complete. Let me explain for those that are not familiar with this. A three pipe loop consists of a common suction, liquid, and hot gas line. A suction stop solenoid is placed at each coil, preferably on top of your cooler box for servicability. A hot gas solenoid is teed in ahead of the suction stop, between the coil and solenoid. When the call for defrost takes place, the suction stop is closed and the gas solenoid is open for the defrost cycle. You can do this for each coil idividually so as not to have overlapping defrost cycles, which will greatly affect your head pressure, especially during colder days. One thing that we like to do, is to put a master hot gas solenoid at the rack and a pump out solenoid back to the suction line to pump out the main gas line to prevent the gas from condensing in the line in between defrost cycles. This can prevent a possibility of hydaulic pressure when the next system goes is defrost due to the condensed liquid in the gas line.

On a parallel system there is no comparison. Hot gas is far more economical and easier to service in my opinion. One timer and a relay with normally open contacts on the hot gas solenoid and normally closed contacts on the liquid solenoid and the suction regulating valve. set the regulating valve to maintain 75 psi(5 bar) when de-engergized and that`s it.

Hmmm. Why go from 35°F to 28°F? Seems like a classic "If it ain't broke, don't fix it" scenario.

Ideal red meat storage is in a high relative humidity, above-freezing environment.

At 28°F you will be freezing surface of meats held in storage. When meats freeze, moisture expands into ice crystals, ruptures cell membranes and upon thawing, meat loses moisture, weight (=$) and flavor. Slowly freezing meat forms larger ice crystals and ruptures more cell membranes. Flash freezing is an attempt to minimize tissue damage by minimizing the sizes of ice crystals.

(Poultry is an exception because it spoils quicker, doesn't lose much flavor, and they package it with those absorbent plastic "spongey" pads that absorb the extra moisture to keep the package weight consistent and your shopping bag from soaking through, usually!)

Also, colder coil surface equals lower vapor pressure in storage room air and therefore more moisture loss except in hermetically sealed product. And in that product, you will now invite the classic "freezer burn" cycle of meat surface degradation as moisture condenses as ice crystals inside the package.

About "resistive vs. inductive (?) watts: with electric defrost, you are "burning" kW's whereas with hot gas defrost, like a heat pump, you're moving heat from one place to another. In this application, you would probably have to defost a single evap fan-coil unit at a time to keep enough load on the system to generate enough hot gas. Failing that, a Kramer Thermo-bank or something similar would be a good idea.

I love hot-gas defrost in theory, but personally have little experience and have heard many caveats regarding compressor problems with poorly designed systems. I'd hate to earn my experience on a system with as much at stake as the one you've described, unless I had a company like Kramer blessing my work.

Rog

Hot gas defrost lines must be insulated to work well. They are harder to defrost back to normal after the cycle messes up on you. Hot gas seems to work better with smaller tubes they use in evaps these days. Most of the time you still need some kind of electric heat to keep the drain flowing smooth. Seems like they always have those pesky areas that never defrost. longer defrost cycles seem to bring more liquid back to compressor. I like shorter defrost times with more frequent defrost cycles.
Hot gas has a better oil return.
That said I think electric is easier to maintain and easier to teach.

Just a few days discovered this marvelous website, therefore a late reply on the discussion hotgas/electricla heaters.
Another phenomen happens when defrosting with got gas (versus electrical hearters EH)
Most of the heaters are fitted into holes of the fins. Few have them mounted in empty tubes (pressed on the fins).
When defrosting with heaters, the heat will be developed between the fins and also between the on the fins formed ice. This means that the ice will be melted from the outside to the inside of the ice. Therefore, a lot of the heat will flow upwards between the not yet melted ice to the sealing an this forms water drops. So a lot of the energy is wasted to heat the ceiling due to convection.
On the other hand, when hot gas defrosting, the ice melts from the inside of the ice to the outside due to conduction. As long as there is ice on the fins, there will be almost no heta losses due to convection.
We measured this once years ago on an evaporator with tremendous troubles with water drops on the ceiliong, finally dripping down on the floor (just in the passage of the reachtrucks) We modified to hotgas and all the problems where gone. Defrost time with heaters +/- 20 to 25 minutes, hotgas max 10 minutes.

I like hot gas defrost when extenuating surcumstances push you that way. I ussually install HG when there is not enough space in the electric service panel. The best part is when I lose the contract and the new people can't work on HG. Makes me look even better than I am.

I do not understand the floating head part. The gas never goes to the condenser. Head will be very low everytime. I must be doing something wrong.

Long defrost times is another area where I am confused. I use short defrost times to prevent liquid slugging with HG.

Hi,

Well, over the past couple of years (since this thread originated), I've accepted as good practice the utilization of auxiliary switches on the compressor contactors to enable and disable heat relays (contactors) for defrost heater control.

As long as the compressor and heaters are not "calling" at the same time, there is no need for additional (seperate) electrical service for the heaters and condensing unit. In my experience, I have not found a low temp system in which the electrical requirements for electric defrost exceeded the compressor's requirements.

Cheers!

Hello,
PHEnergy here, I'm new! and I only got into the Refrigeration Industry five years ago because it consumes so much Electrical Energy, and my Company reduces Electrical and Natural gas Bills for high Energy consumers. :)
Since Electric Heaters Consume less Electrical energy than the Condensing Unit, Which MUST run to provide the Hot gas I tend to go for electric. It is also easier to control.

But I would like to start a new thread here, one off my sub contractor's tech, just added Capella Oil (Texaco 68) to a newly commisioned blast freezer system using R404A, I Believe there are serious problems to follow. He says he will convert to the more expensive oil POE after a week, is R404A compatible to Capella? if not, how do we go about correcting the error, will we lose all the newwly installed R404A gas? the system demanded 9 x 24Lbs Cylinders of refrigerant. I am now seeing a dark brown cloud in the upper third of the sight glass. the system was started only Yesterday.

Hi PHEnergy.

Texaco 68 is air compressor oil

Look here and see the mistake you have made.

http://www.selectlubricants.com/manufacturers/Texaco/

POE was the right oil for the job.

Chemi :)

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will we lose all the newwly installed R404A gas?

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I think the answer is YES. :eek:

And it will cost you to clean the system. If you stop now you may not lose the compressor but - hey, no promises.