to pump down or not to pump down

[Toolman]

In my experience I'd personally prefer a CPR on a system than an MOP valve, but the systems we install rarely see either and are more often than not rack systems. An MOP shuts on pressure - where its quite possible the comp would not be pulling max amps in that case, at least with the cpr you know you are limiting system performance ONLY when its required.
The manufacturers concerned (Buffalo and Heatcraft) do thorough R&D testing - they don't print these things for the fun of it (got a good laugh out of that post!)

Good to see I got a laugh 750

So its settled then .....not by the look of this thread , is there any area of Fridgy work that MUST be done only one way ?
My statement still stands about leaving gas in coil to assist transfer heat during defrost . But yes shutting down a soleniod on the liquid line at the same time as the defrost initiates may help prevent migration , maybe I'll do that on the next job.
MOP valves are OK but what happens when some new guy comes along and its 2am on a breakdown and he only has a standard TX valve , he going to fit it isnt he ? And its unlikely he coming back the next day to fit the MOP valve so the protection giving by the MOP is removed from the system fully unless its a multiple system .
At least with a CPR you know its there and not going to be removed ( unless someone is really stupid ) and it will choke back the high pressure at start up after a defrost .
Anyone who trusts a wholesaler to do there full engineering is on thin ice , ultimate they just want to sell you the stuff , when it doesnt work they run a mile ( they certainly dont pay for the labour to fix it or defrost it ), notice on there invoices it says something like " we take no responsibility for any equipment sold or engineering on anything installed " Or something like that .

[750 Valve]

Fully agree on the MOP thing, but i have no patience for ill equipped service blokes - they get the callout fee for that late nighter - and get a good rest the next day, meanwhile someone else goes back and actually fixes the problem. If it needs an MOP they can go get one from a wholesaler - no matter what the time of day!

Also agree on getting someone else to engineer your installations, they are only half a mechanic if they can't select their own gear - nobody's asking them to design the coil - just pick the right one for their customer.

[jwasir]

Hi,

Mostly discussion is about using CPR for after the defrost conditions.

I would like to know, if we can eliminate CPR for During Hot Gas Defrost conditions for a SINGLE evap.?

I've seen that sometime during Hot gas defrosting,when coil temp is satisfied and suction pr. increases, this increases the discharge too high and system goes on HP cutout.

Any ideas, how to eliminate these conditions during HG defrost, by NOT using CPR.

[taz24]

Hi,

Mostly discussion is about using CPR for after the defrost conditions.

I would like to know, if we can eliminate CPR for During Hot Gas Defrost conditions for a SINGLE evap.?

I've seen that sometime during Hot gas defrosting,when coil temp is satisfied and suction pr. increases, this increases the discharge too high and system goes on HP cutout.

Any ideas, how to eliminate these conditions during HG defrost, by NOT using CPR.

No I don't think so.
With one evap the normal way to defrost with hot gas is to inject the hot gas into the evap just after the TEV.
That has the disadvantage that when the hot gas is injected into the evap the evap pressure go'es through the roof. If this is alowed to continue then the comp will overheat. The hp will not rise too much, acctualy it will drop because the discharge is now directed through the evap without the pressure drop of the TEV. If this cycle is allowed to run for too long a period of time then the comp will trip on thermister or klixon. A good controler that measures the actual temp of the evap and then terminates on temp and not on time will stop the problem.

taz.

[jwasir]

THANKS TAZ,

Sometimes I noticed that before the controller terminates the defrost, the head goes really high and system cuts off from HP cut out.

Then we installed CPR, to choke the suction pr.

My question was how others tackle this kind of scenario OR do we always need CPR for single evap HG defrosting.

[old gas bottle]

might be able to sort this out by fine tuning the defrost terminations, somtimes on hot gas it blasts the ice off very quickly and then carries on causing the trip problem, shorter defrost times and longer drip down will help, CPR,s are a good tool and i prefer to fit them but as controlers get more functions the less applications they are needed, not like the old days where you just had a clock

[WVDM]

^ the engineer i spoke to wasn't suggesting that you omit the cpr, he was just saying that bitzer had told him it "could" be done. it makes you wonder how far their tolerances go.

YOU GET COMPRESSORS THAT ARE DESIGNED FOR PULL DOWN THEREFOR A WIDER ENVELOPE EG STARTS @ +10 to -35 evaporating these compressors dont need cpr or mop valves the motors can handle the load generally dont get as much kw out of these as apposed to narrow envelope compressors which are generally used for holding freezers but can be used FOR PULL DOWN SO MOP VALVE OR CPR A MUST

[jwasir]

Is there any HG defrost valve a/v which acts as MOP valve during defrost?

[benj2212]

sorry if i'm repeating others, my opinoin is electric defrost evaporators should be designed to defrost the coil properly without gas or any other conductors. most of the newer systems produced are getting poorer in quality and cannot handle high startup pressures. assuming the freezer room mentioned in the first post is a regular food storage room then it wont have any liquid acumulators or a crank case heater so leaving liquid refrigerant in the system could cause the compressor oil to froth. pump down is simple and cheaper provided it is correctly designed for the purpose.

[750 Valve]

sorry if i'm repeating others, my opinoin is electric defrost evaporators should be designed to defrost the coil properly without gas or any other conductors. most of the newer systems produced are getting poorer in quality and cannot handle high startup pressures. assuming the freezer room mentioned in the first post is a regular food storage room then it wont have any liquid acumulators or a crank case heater so leaving liquid refrigerant in the system could cause the compressor oil to froth. pump down is simple and cheaper provided it is correctly designed for the purpose.

Man it must be cut throat where you are, the systems are only getting poorer in quality as the contractors are omitting accumulators and c/case heaters to save a buck and win a quote or boost profits. I RARELY see LT systems without accumulators - and NEVER see LT systems without c/case heaters, thats just being bodgey.

I find it amazing how we forget all the things we learnt over the years and just let the almighty dollar drive what we do - I have this beef with our engineering dept all the time, seems they have forgotten what an EPR is and think you can cycle 48ft of case on discharge air temp...

[Dr._Fleck]

Well said Benj, i agree totally.

Some people on here have some strange ideas.

[benj2212]

the common systems for freezer rooms here are l'unite hermetique which have no crankcase heater or suction accumulator mainly for cost cutting which seems the norm now. these systems work ok but dont last for long, in fact most compressors dont last 3 years for obvious reasons. cheers dr fleck. seems we cover the same area. is that rm from you avatar. im working for a blackburn based company but always looking for brighter horisons

[750 Valve]

Well said Benj, i agree totally.

Some people on here have some strange ideas.

Strange ideas - like doing things properly? Where I work you'd be called "rough" and not get paid much for your (lack of) skills. I guess if you don't understand what you are doing then ignorance is bliss, good luck mate - just keep puttin those splits in and don't dare to think about what you are doing or saying, it might hurt the old grey matter

Like you said benj2212 its all about cost cutting, thats not a valid reason IMO. Our units don't come with these things as standard either - we see a lot of l'unite hermetique stuff here too, you have to add them to the order and sometimes (I know this is a strange idea for blokes like fleck) you actually have to pipe the accumulator in and fit the heater yourself

[powell]

Interesting thread. I've not heard of this theory of defrost.

Pump down prevents liquid migration and also floodback on start up for coolers and freezers.

Maybe they have designed a better mouse trap but I wouldn't buy into this procedure until actual field experience proves me wrong. But that's just my opinion.

A LLSV has a small price to pay for the protection it provides.

Does the evap mfg claim that installing a LLSV hinders the defrost in their coils? Or is it a scheme to claim lower installation cost and fewer joints to leak.

I wonder what all the compressor manufacturers opinion might be?

[Dr._Fleck]

Yeah whatever.


Ben i've sent you an email.

[superheated]

I am thinking about using "pump down" on a cap.tube system. Would it work properly as a system with a TX valve?
Compressor:1.5 hp semi-hermetic low temp. 404A
Defrost: Electric.
Condenser: air-cooled

[Dr._Fleck]

Yes it will work ok. You will have to alter the wiring though, so that the stat operates the mag valve and the LP operates the contactor.

[superheated]

thanks Dr. Fleck,
I'm a bit concern about the condenser's capacity to handle the refrigerant during pump down, since I am not adding a receiver to this system.

[750 Valve]

Interesting thread. I've not heard of this theory of defrost.

Pump down prevents liquid migration and also floodback on start up for coolers and freezers.

Maybe they have designed a better mouse trap but I wouldn't buy into this procedure until actual field experience proves me wrong. But that's just my opinion.

A LLSV has a small price to pay for the protection it provides.

Does the evap mfg claim that installing a LLSV hinders the defrost in their coils? Or is it a scheme to claim lower installation cost and fewer joints to leak.

I wonder what all the compressor manufacturers opinion might be?

Both mfg's stating this also mfg compressors, one is Heatcraft and the other is Bitzer. They don't care if you fit a solenoid, it doesn't hinder defrost and has no bearing on their installation costs - they aren't installing them, thats up to the contractors who buy them. They just state that pressure should be allowed to remain in the coil to aid in the heat transfer while on defrost, just like hot gas systems where the ice will melt off the pipe first as the heat transfers out from the pipe containing the hot gas. They are claiming that a more even coil temp can be obtained when defrosting by this method. Just because there is pressure in there doesn't necessarily mean it will cause issues on startup, a LLSV is still good to use as I stated before but can be cycled with the comp to stop migration.

[powell]

Both mfg's stating this also mfg compressors, one is Heatcraft and the other is Bitzer.

750,

That's interesting about Heatcraft. One of my business friends was recently in Austrialia for three weeks. He told me that Heatcraft had their on supply houses for the HVACR trade.

They must have a license with the compressor manufacturers to build the compressors. Is it Copeland?

Thanks...............Powell

[Dr._Fleck]

From my experience it's better to fit an LLSV. What's £50 compared to having to go and replace a compressor that is still under warranty.

Also think of this analogy:

Copper is a good conductor of heat the same as refrigerant, agreed? So, imagine an evaporator made of SOLID copper, solid copper sat at -30c at the start of a defrost. Would this solid evaporator defrost faster than a normal empty tubular one? The extra MASS of the solid one would take longer to defrost, all the extra mass would have to be slowly pulled up to 1c or above before the defrost can end.

Someone mentioned:
"just like hot gas systems where the ice will melt off the pipe first as the heat transfers out from the pipe containing the hot gas"

... i'm trying not to laugh... Hot gas defrost is something totally different.


Superheated. Consider the length and size of the liquid line and size of the evaporator, as this will tell you how much liquid there is to pump down.

[750 Valve]

Someone mentioned:
"just like hot gas systems where the ice will melt off the pipe first as the heat transfers out from the pipe containing the hot gas"

... i'm trying not to laugh... Hot gas defrost is something totally different.

Really mate - take something out of context and you can twist it to make any point you like. How does a solid copper evaporator has ANYTHING at all to do with this argument? Where did you pluck that one from? Try to think before you type mate, you obviously can't understand what I'm saying so leave it at that or try to read the WHOLE thread and not just jump in on the last post.

I talk to the engineers at the manufacturers in question and know what R&D goes into it, you haven't the slightest idea and just keep banging on about a LLSV - you obviously didn't read the entire thread - I suggest you do, maybe it might make sense to you then and you may just even learn a thing or two.

Powell - Heatcraft bought a large australian manufacturing company called Kirby refrigeration a few years ago which made their own coils, units and various other equip, they also had a manufacturing arm called Kulthorn Kirby that manufactured compressors - Heatcraft now owns them. They put Copeland semi's in their units but make their own hermetic domes, scrolls and rotary comps. The old Kirby wholesale outlets are now all branded Heatcraft.

superheated - not sure why you would want to pump down a cap system but a cap system should have its "liquid line" kept to a minimum, lots have the cap basically straight at the condenser outlet just after the drier - this helps to keep a balanced charge and not stop floodback on startup - so finding a location may not be easy. If your system is experiencing floodback on startup (the only reason I can think of to pump down a cap tube system) there may be something else at fault - overcharge, bad design - eg long liquid line, no accumulator. If i had to fit one I would be tempted to put it just before the cap inlet then that should eliminate any liquid lines to pump down (as the above poster mentioned) and increase the volume available to store the charge. You can also look at setting the LP cutout a little higher than say a normal pumpdown system so as to not push the envelope with bumping all the charge into the condenser.

[Dr._Fleck]

I have read all the posts. I'm going to bow out of the debate now like all the other clued-up engineers have politely done.

What we have here is a religion, a blind belief in an idea with absolutely no statistical data to back it up.

Amen

[750 Valve]

I have read all the posts. I'm going to bow out of the debate now like all the other clued-up engineers have politely done.

What we have here is a religion, a blind belief in an idea with absolutely no statistical data to back it up.

Amen

You go do your reading then, if you are keen speak to the manufacturers specifying this - I am not one of them I just understand what they are saying. There is more than 1 way to acieve a result in this industry and you shouldn't be afraid to learn something new, it may not be for you but thats no reason to dismiss it - especially if you haven't the slightest idea about the specifics being discussed.

I'm off to church now Then i have to find my solid copper evaporator I made last year and do some testing...

[The MG Pony]

From my experience it's better to fit an LLSV. What's £50 compared to having to go and replace a compressor that is still under warranty.

Also think of this analogy:

Copper is a good conductor of heat the same as refrigerant, agreed? So, imagine an evaporator made of SOLID copper, solid copper sat at -30c at the start of a defrost. Would this solid evaporator defrost faster than a normal empty tubular one? The extra MASS of the solid one would take longer to defrost, all the extra mass would have to be slowly pulled up to 1c or above before the defrost can end.

Someone mentioned:
"just like hot gas systems where the ice will melt off the pipe first as the heat transfers out from the pipe containing the hot gas"

... i'm trying not to laugh... Hot gas defrost is something totally different.


Superheated. Consider the length and size of the liquid line and size of the evaporator, as this will tell you how much liquid there is to pump down.

WTF?

Me thinks you aut to have another read befor the daily brew, as the above makes not one tad bit o sense!

[palmerm]

Pump down does not mean you get rid of all the refrigerant in the coils. All decent engineers advocate leaving refrigerant in coils but this doesnt mean you cant pump down. CPRs should always be used to eliminate possible compessor overloading - cheap insurance.