to pump down or not to pump down

[Temprite]

G'day all.

When we do an install of a new walk in freezer we always make it a pump down system with an oil trap if condensing unit is higher than evaporator.

Having said this I was reading some information supplied by manufacturers of evaporators that are very popular in Australia, which read as follows.

Do not make the system a pump down system as the defrost requires refrigerant to be in the coil to distribute the heat evenly throughout the coil and allow an adequate defrost. Suction line must exit the evaporator and go straight up to prevent refrigerant draining on a defrost cycle, causing poor defrost.

Just interested on other members opinions on this.
Thanks in advance.

[taz24]

as the defrost requires refrigerant to be
Just interested on other members opinions on this.
Thanks in advance.

It would depend on the compressors ability to strart under load.
If the comp is rated for the load then why not?
If the comp won't then put a cpr in and that will take care of the startup pressure.

taz.

[Samarjit Sen]

The opinion of the Evaporator manufacturer is correct. As taz24 says, I would advise to use a cpr for a hot gas defrost.

[Temprite]

The opinion of the Evaporator manufacturer is correct. As taz24 says, I would advise to use a cpr for a hot gas defrost.

hello samarjit

this is electric defrost. not hot gas.

regards

[smpsmp45]

In almost all the units which we have used either from EFC, Spain & Zanotti , italy, Monoblock / split units, are using Pump down by default on all units

[Grizzly]

G'day all.

When we do an install of a new walk in freezer we always make it a pump down system with an oil trap if condensing unit is higher than evaporator.


Do not make the system a pump down system as the defrost requires refrigerant to be in the coil to distribute the heat evenly throughout the coil and allow an adequate defrost. Suction line must exit the evaporator and go straight up to prevent refrigerant draining on a defrost cycle, causing poor defrost.

Just interested on other members opinions on this.
Thanks in advance.

I must be thick because I cannot see any reason why you would want to have an evaporator full of refrigerant except for when your in cooling mode.
When on defrost surely if the evap was full of refrigerant, the bulk of the heat from the heater rods. Would be absorbed by the refrigerant it is "boiling off".
Secondly on start up you have an evap full of refrigerant and nothing between it and the Compressor. Except the gravitational pull required to overcome the Vertical riser!
Sounds like a recipe for disaster to me.
Or yet again am I missing the point Guys?
Steve.

[Temprite]

www.actrol.com.au/install/buffaloBRLE.pdf

see page 3

[old gas bottle]

i,am with grizzly on this one,sounds like a weird explanation of how things should be to meheating up a evaporator full of gas/liquid and oil seems to me pointless and possibly harmfull to the compressor aswell as wasting energy cooling it back down again, and as far as the suction line goes,most go up before going up or down again,and as far as draining the refrigerant out,! well if it was pumped down that would not apply anyway,

its been industry standard to pump down for as long as i can remember on heated evaporators and has allways served me well and most of the world too, so i,am not changing unless there is a very good reason.

[Grizzly]

www.actrol.com.au/install/buffaloBRLE.pdf

see page 3

Temprite.
I see it but I dont believe it!
your absolutely correct to question it. But I don't, like you, know why they advocate it.
Personally Before I fitted their evaps I would want to speak to them.
Just dosn't add up.
Cheers Grizzly

[Temprite]

Yes strange indeed.

They must want you to use a CPR instead of pumpdown. Most other fridgys I know use pump down method.

I am going to contact the manufacturer for an explanation, and will post it here.

[Taaz1275]

Quite interesting ...lets take a look at this Patton CL, CM, and CH model condensing units - semihermetic DWM Copeland compressors...in relation to your question, pump down system is used in most freezer systems during defrost cycles but in the case where pump down system is not used to prove theory on the need for refrigerant to be in the evaporator coil for more efficient defrosting ...the Patton model condensing unit that I mention above gives us a more understanding to your question ...where they use a CPR before the suction accumulator plus the inclusion an avaporator fan time delay after every defrost or during start ups....this I believe will prevent high current start ups...and floodbacks after defrosts.

[old gas bottle]

just pose the question, what would be the effects on systems designed for low temperature where the compressor set is a LT model,without all the bits added like a CPR,intelligent defrost system,etc, the compressors would not last, it would need to be a ht/mt/lt model.
you can just picture it, hot day,sun baking the condensing unit while its off on defrost, starts up,massive workload to overcome, pop goes the hp,game over.

[Grizzly]

Quite interesting ...lets take a look at this Patton CL, CM, and CH model condensing units - semihermetic DWM Copeland compressors...in relation to your question, pump down system is used in most freezer systems during defrost cycles but in the case where pump down system is not used to prove theory on the need for refrigerant to be in the evaporator coil for more efficient defrosting ...the Patton model condensing unit that I mention above gives us a more understanding to your question ...where they use a CPR before the suction accumulator plus the inclusion an avaporator fan time delay after every defrost or during start ups....this I believe will prevent high current start ups...and floodbacks after defrosts.

Taaz
A evap fan time delay should be included irrespective of defrost type. Also known as drain down time.
1) To allow residual heat in the evap to disapate locally prior to the fans starting and blowing warm air arround the room.
2) To allow the water produced to drain down and not be refrozen in the drip trays and drain lines.
Grizzly

[yusuf amur]

hay..it is depend for such unit which the condens unnit its level above the evaporatore system should install with oil separetor . pumpdown dose not effect the diffrost cylce ,but its use to stop the compressor through low presure controll only if room temperature rich the limit and pumpdown used to emty ***** from evaporator and other part of low side of the system .difrosst by means of hot gas or electric it operat during the defrot cycle normaly for hot gas,condesor fan stop evaporater fan stop selonoid valve pass the hot gas to the evaporator
for electric difrosst compressos stop con fan stop eva fan stop both evaporator heater element with drainage heater operate .that is to say two diferent between pumpdown and deffrost thank you.


Having said this I was reading some information supplied by manufacturers of evaporators that are very popular in Australia, which read as follows.

Do not make the system a pump down system as the defrost requires refrigerant to be in the coil to distribute the heat evenly throughout the coil and allow an adequate defrost. Suction line must exit the evaporator and go straight up to prevent refrigerant draining on a defrost cycle, causing poor defrost.

Just interested on other members opinions on this.
Thanks in advance.[/quote]

[Toolman]

Having said this I was reading some information supplied by manufacturers of evaporators that are very popular in Australia, which read as follows.

Do not make the system a pump down system as the defrost requires refrigerant to be in the coil to distribute the heat evenly throughout the coil and allow an adequate defrost. Suction line must exit the evaporator and go straight up to prevent refrigerant draining on a defrost cycle, causing poor defrost.
.

Temprite as you know I'm in OZ and we never use a Soleniod pump down system on a freezer room BUT we use a soleniod to shut off the liquid line during defrost to prevent migration of more liquid than what is in the line

The manufacturer is correct ( whoever it is ? B/T or Kirby )

The retaining of refrigerant in the evaporator is a good thing as it transfers the heat from the heaters through the coil . ( as we know refrigerant is the best heat transfer medium ) so dont fit a soleniod just a CPR and suction accumulator .
We have many freezer rooms like that and no problems . The CPR will cover any comps that dont like starting under load plus it will restrict the initial high pressure when the system re-starts .

[Samarjit Sen]

Thanks for the good and valuable information. We have been using pump down for electric defrost, and had some problems. Now I understand the reason. If possible could the link to the original manual be provided on the forum.

[Temprite]

Temprite as you know I'm in OZ and we never use a Soleniod pump down system on a freezer room

The manufacturer is correct ( whoever it is ? B/T or Kirby )

The retaining of refrigerant in the evaporator is a good thing as it transfers the heat from the heaters through the coil . ( as we know refrigerant is the best heat transfer medium ) so dont fit a soleniod just a CPR and suction accumulator .
We have many freezer rooms like that and no problems . The CPR will cover any comps that dont like starting under load plus it will restrict the initial high pressure when the system re-starts .

Its a buffalo trident. CPR or pumpdown either method suits me.

Funny thing though when you talk to some fridgys they would rather cut their own arm off than use a CPR.

Toolman from your experience do you reckon the CPR fitted systems defrost better than pumpdown system?

Cheers

[Toolman]

Its a buffalo trident. CPR or pumpdown either method suits me.

Funny thing though when you talk to some fridgys they would rather cut their own arm off than use a CPR.

Toolman from your experience do you reckon the CPR fitted systems defrost better than pumpdown system?

Cheers

Thats nice for once I'm being asked advice.

CPR is always the go , like I said earlier you need that refrigerant in the coil to transfer the heat through the coil during defrost , you would only use a soleniod to control the temp of one item if you had multiple medium temp systems on one compressor.
If someone says the compressor cant start under load they havent set up the CPR to choke off the pressure at start up . Also a CPR needs no power supply , sure they cost a little more than a cheap soleniod but no power supply is required and one return call from an ice up ( using a pumpdown system ) and the money you tried to save is lost .

They would cut there arm off ? Sounds like a defence to not understanding something
Google it = crankcase pressure regulator for freezer

[US Iceman]

Do not make the system a pump down system as the defrost requires refrigerant to be in the coil to distribute the heat evenly throughout the coil and allow an adequate defrost. Suction line must exit the evaporator and go straight up to prevent refrigerant draining on a defrost cycle, causing poor defrost.

Well, that's a new one for me!

If the defrost is accomplished by electric heat, the heat is conducted through the evaporator through the tubes, then the fins. As the evaporator starts to warm up and refrigerant vapor left in the evaporator would start to superheat, That does not mean the refrigerant heats the coil surfaces, but rather the opposite. Unless hot gas defrost is being used...refrigerant has nothing to do with defrosting a coil.

The use of a CPR is determined by the need to limit the compressor amps & high discharge pressures during pull down, unless something new has occurred that I'm not familiar with.

I would always suggest a pump-down cycle to remove as much refrigerant from the evaporator during off-cycle or prior to defrost.

And, if the condensing unit is located at a higher elevation than the evaporator an oil trap should be installed on the evaporator suction line. You might want to try to limit the volume of the trap so too much oil does not hang-up in it though.

[Toolman]

Theres always many opinions - Buffalo Trident ( the evap manufacturer ) dont have any interest in making there evaps defrost properly they just write stuff down for fun .

[US Iceman]

Buffalo Trident ( the evap manufacturer ) don't have any interest in making there evaps defrost properly they just write stuff down for fun .

I'm not quite sure how to take that comment, but I doubt they write down just anything to fill up a page.

I did read the comments in the instructions provided, and will say again, this does not make any sense.

I'm open for a discussion on why the refrigerant is needed to defrost the evaporator coil though. Opinions have nothing to do with it. If it's based on sound principles then it works.

And as for the P-trap not being required, the evaporator is already trapped by the suction line riser going up.

The text pertaining to defrost sounds like it was intended for hot gas defrost, not electric.

[centrifs]

I am with iceman on this.
1) if you pump down an evap before defrost, it still has refer in it, at a lower pressure than you down unders are talking about, but refer to aid in heat transfer ?? the metal to metal contact of htr to fin is the best heat transfer you can get
2) if cond is above evap always use pump down and trap to return oil to comp, which is most likely to get caught up in evap when evap is iced and needing defrost
3) if cond is below evap inverted trap and pump down to avoid liquid draining to compon off cycle.

just a thought for you, if not pumped down, during defrost you superheat all that gas in evap. what do you think happens at start of cooling with fan delay for drip down, all that SUPERHEATED gas running back thru nice well insulated suction lines at say -20 deg c.
a )it stays superheated gas
b)it condenses to a liquid
c) dont know, never thought about it, dont care and am going to do what the manual says!!

[fri11j]

I always use liquid line pump down on low temp, combined with a CPR valve and trap on evap.
I exclusivley use Buffalo-trident evaps for all my cool room-freezer room applications and have never seen ice up problems relating to a starved evaporator on pump down.
The buffalo evaps have the heater rods directly through the centre of the evap assembly, unlike some that have the heaters clipped on the front and rear of the Evap.

The BIG question is... how do you calibrate your CPR valve???????
To you adjust it to control suction pressure or tong the compressor and use it to regulate your Full load Amps ????????
So many confilicting views on this one but I use it to regulate FLA...

[Grizzly]

I always use liquid line pump down on low temp, combined with a CPR valve and trap on evap.
I exclusivley use Buffalo-trident evaps for all my cool room-freezer room applications and have never seen ice up problems relating to a starved evaporator on pump down.
The buffalo evaps have the heater rods directly through the centre of the evap assembly, unlike some that have the heaters clipped on the front and rear of the Evap.

The BIG question is... how do you calibrate your CPR valve???????
To you adjust it to control suction pressure or tong the compressor and use it to regulate your Full load Amps ????????
So many confilicting views on this one but I use it to regulate FLA...

Fri11j
Thanks for your post. We will get to the bottom of this eventually.
Would it be possible for someone to ask Buffalo Trident for a simple answer as to why they state what they do.
Put us all out of our misery please cause as you Aus
Guys state they must have a reason. I just can't see it?
Personnally I don't know of a evap manufacturer who dosn't pit the heater rods through the centre!
There are quite a few who also put periferal heaters on the outside. A round the evap fan cowls and in the drip trays.
I am supprised that so many advocate CPR Valves though as it's not an energy efficient way of controlling a Compressor.
As your post eludes to fri11j
Toolman
I have no real problem with your explanation of using a CPR Valve.
It's the evap full of liquid to evenly spread the heat
I can't understand?
I suspect that as I say without the Manufacturers explaination we will flounder.
you would only use a soleniod to control the temp of one item if you had multiple medium temp systems on one compressor... If you mean to each Evaporator then I agree!
What temperature is Medium??
No one has actually said what temps we are discussing
Grizzly

[Dr._Fleck]

I'm with the pumpdowners. If there is liquid refrigerant in the evap during defrost, this dense liquid will have a large mass that will take more heating up, and will prolong the defrost. Some of it will evaporate and hold the temperature down until the pressure goes up... it's all bad news, don't even go there, pump it down.

As for refrigerant being a good heat conductor, i boubt it is better than copper.

[US Iceman]

To you adjust it to control suction pressure or tong the compressor and use it to regulate your Full load Amps ????????
So many conflicting views on this one but I use it to regulate FLA...

There is small penalty for using a CPR because of the pressure drop created by the valve itself. You have to pay for this all the time. The purpose of a CPR is to prevent overloading the compressor (amps) & condenser (heat rejection) when the suction pressure is higher after defrost (on electric defrost units).

To set it, use the desired suction pressure at the compressor plus a small allowance. You don't want the suction pressure too high (for the above reasons) but you want the valve fully open at the normal operating suction pressure. By using a small pressure allowance this allows the compressor to pull the load down a little faster.

[Grizzly]

www.actrol.com.au/install/buffaloBRLE.pdf

see page 3

OK Guys the plot thickens. I have just read the pages supplied with the link sadly the commitioning data is missing.
But a few points yet to be discussed.
These evaps are designed for a -35c to 0c range. With an orifice plate installed within the distributor.
Externally equalised TXV are required with a Defrost termination stat set to 13c + or - 2c.
Also a fan delay set to 2c = or _ 2c.
1) STOPS THE DEFROST
2) WONT ALLOW THE FANS TO RUN UNTIL THE CORE TEMP IS DOWN TO 2c approx.
Also the room stat is wired into the Compressor O/Load circuit. So when the room reaches temp the compressor stops. (Last time I saw that the liquid flood back was horrendous.)
Am I correct in thinking with this control stratergy!
and a CPR the evap would pump down on compressor startup. And then everything would balance out and everything would be as would be expected?
Any ideas anyone?
Grizzly

[lana]

Hi Everybody,

Very interesting?!

I am supporting US Iceman's comments. Refrigerant has nothing to do with defrost or heat distribution. If there is refrigerant present then high pressure would cause more damage.
If your evaporator and compressor allows to do pump-down then this is the way.

I was reading an article from Trane about oil traps which was really embarrassing for a company like that to write such an article.
The author says that "using oil traps is old fashioned. Now a days we don't use oil traps."

Can you imagine a company like Trane claims something like this?

Cheers

[nike123]

I was reading an article from Trane about oil traps which was really embarrassing for a company like that to write such an article.
The author says that "using oil traps is old fashioned. Now a days we don't use oil traps."

Can you imagine a company like Trane claims something like this?

Cheers

Yes I can, if it is in right contest. In low temperature applications that is wrong, but in air conditioning that statement is pretty much OK!

[old gas bottle]

perhaps now with new refrigerants and oils there are instances where they are not needed as often, but my view is they seldom do any harm and often do a deal of good, untill we stop using oil i will continue to fit them in the traditional manor.

[nike123]

perhaps now with new refrigerants and oils there are instances where they are not needed as often, but my view is they seldom do any harm and often do a deal of good, untill we stop using oil i will continue to fit them in the traditional manor.

Doesn't much matter is it new or old refrigerant or oil. It matters refrigerant quantity and speed threw pipes and solubility of oil in refrigerant. If you keep proper speed and density of refrigerant threw entire length of pipe, oil doesn't have chance to accumulate. Of course if you have extreme situations you should consider adding of oil traps.

[Dan]

Regarding liquid refrigerant in evaporator coils assisting in defrost. Hussmann tub cases with electric defrost, the engineers recommended leaving liquid in the evaporator to shorten the defrost duration. It makes some sense if you think about it. A heater on the face of the evaporator will boil the refrigerant away, assisting the defrost as it travels to the deeper passes and condenses. Thus you have the internal fluid assisting the external convection of the air. Whether this applies to off time defrost, I think it would be less so and perhaps not worth the sacrifice of a pumpdown system.

[dsp]

Hi Guys,
I work for a wholesaler and always qoute for and recommend pumpdown systems on above applications. So do the engineers I work with. As for Buffalo stating to leave refigerant in evap. during defrost, I wouldn't recommend it without more reasoning. Look forward to asking someone at Buffalo or Actrol with this one.

[US Iceman]

A heater on the face of the evaporator will boil the refrigerant away, assisting the defrost as it travels to the deeper passes and condenses.

If the heater is energized and providing heat to the evaporator surface, any liquid refrigerant remaining in the coil would boil, which would tend to maintain a constant temperature, hindering the defrost.

Of course, any refrigerant vapor in the coil could condense in the coil if it came into contact with any metal below the saturation temperature of the gas. But then we have liquid again and need to drain it out of the coil, hence the trap requirement.

[lana]

Hi everybody,

Doesn't much matter is it new or old refrigerant or oil. It matters refrigerant quantity and speed threw pipes and solubility of oil in refrigerant. If you keep proper speed and density of refrigerant threw entire length of pipe, oil doesn't have chance to accumulate. Of course if you have extreme situations you should consider adding of oil traps.

There are two situations :

1- System running : when compressor is on and the suction (or other) riser is properly sized then oil will go up in a vertical pipe and there will be no problem and oil traps do nothing. Horizontal pipes must be pitched (1%) towards compressor (suction) and towards condenser (discharge).
OIL TRAPS DUTY IS WHEN THE SYSTEM IS OFF.

2- System is off : when compressor stops then all the oil which is on the internal surface of the riser will drain down and goes into the evaporator. This happens whether the pipe size is correct or not.

YOU can see oil traps do the work when the compressor is off, NOT when the system is running. It is stated in every proper reference.

Cheers

[old gas bottle]

hey mr nike 123 !yes it does matter, R22 was notorious for poor oil return on systems and some of the older guys used to add a drop of R12 to assist,not the best practice i admit but it used to be done, oil traps and seperators were important and as the old mineral oil aged even more so,and even by changing the compressor oil you could still get oil return problems.

the rest of it i stated,with new gasses and oils its no so common,meaning yes of course with smaller pipework, higher velocity and working pressues,better qualiy oil,the need for traps and seporators on some applications is not so critical,and as i also stated,it does not do any harm to install traps.

[Taaz1275]

Taaz
A evap fan time delay should be included irrespective of defrost type. Also known as drain down time.
1) To allow residual heat in the evap to disapate locally prior to the fans starting and blowing warm air arround the room.
2) To allow the water produced to drain down and not be refrozen in the drip trays and drain lines.
Grizzly

Mr Grizzly,

Thanks for correcting my english as to the word
"inclusion of the evaporator time delay" , I'am aware of the the 2 reasons that you've mentioned, but I was just trying to put this method in relation to the controls of not having a pump down system but the need to install a CPR and a right sized suction accumulator in between the evaporator coil and the compressor. In other words, during this time delay after the termination of defrost,the evap fans remain "OFF" ,the return pressure to the compressor with the CPR in the way should be minimised for as long as the delay timer allows it to,..this is due to the actions of the expansion valve with the evaporator fans not running.This will give enough time to get rid of most of the liquid refrigerant in the evaporator coil when it went on defrost. After defrost:
Yes, there will be hell of floodback as can be seen observed by the frosting up of the suction pipes right up to the inlet of the compressor, but then , the suction accumulator is there to prevent liquid from getting into the compressor, and the CPR to maintain the pressure that is entering into compressor keeping in to a safe running amperage as to the compressor motor during start ups. I believe that the length of delay time is also important in this matter concerning this type of system.
I guess you understand more and could correct me on where you think I might be confused in.

Thank you.
Taaz

[750 Valve]

Guys I'll try to help clear this up, the manufacturer - Buffalo Trident (a division of Bitzer - well, sort of) specifies that refrigerant be left in the evaps for the reasons mentioned above - the increae in pressure also equates to an increase in the coil surface temp and helps to evenly melt the frost/ice from the coil face - the electric elements are not in contact or in close relation to EVERY tube of the coil so this does help
To meet the mfg's defrost times (they pride themselves on never requiring more than 20 mins to defrost any of their coils - be this good or bad) you should adhere to the specs of leaving pressure in the evap.
Now leaving pressure in the evap does not necessarily mean you will start up with a gut full of liquid when the comp cycles on, nore does it mean you have to go to exhorbitant lenghts to stop comp motor overloading, I work for pretty much the largest supermarket company in Aus and we have used only Buffalo evaps for the last 6 years straight - here is what we do.
Fit a LLSV and cycle it off with the comp on the defrost timer - simple and effective. It is not a pumpdown system, the comp cycles on LP or T'stat - whatever you prefer. But the defrost timer should cut the comp control circuit and also shut the LLSV, this will leave an amount of refrigerant to expand in the evaporator, not enough to kill a comp on startup, especially if you fit the LLSV at the room - but we commonly fit them in the plantroom and see no issues at all. If its a rack system we will fit both suction and liquid solenoids (formoing a coil lockdown) with a high pressure bypass in case the heaters get locked on.
Starting a small comp with a higher suction pressure provides less load (current draw) than a high pressure difference and the comp will start easier, as discussed a CPR can be fitted - they are not that scary, or an MOP valve can be fitted. Personally on a single system we do neither but we always use semi's not domes as the loads are typically a bit bigger than your avg ma and pa's deli freezer.
Hope this helps

[The MG Pony]

Thats what I was thinking, as heat rose so would the pressure raising the the temp at which the refrigerant would condence! Thuse providing a more therough defrost, as the refrigerant would boil and condense at a higher temp yeilding this energy to the upper coils.

In a critical charge system the bulk of the refrigerant would be in the condencer and LL.

On start up the compressor would experiance a no load condition untill the pressures ballance?

Would any one agree with this idea? Or am I messing up some where?

[750 Valve]

In a critical charge system the bulk of the refrigerant would be in the condencer and LL.

On start up the compressor would experiance a no load condition untill the pressures ballance?

Would any one agree with this idea? Or am I messing up some where?

Thats the way I see it

[Magoo]

use a liquid solenoid valve and cycle with compressor, stops liquid migration in off cycle, stop compressor on temp control, size CPR on pressure drop ( minimum ) not pipe size and system will perform great. So long as system has been designed with system balance.
magoo

[tonyhavcr]

I'm not quite sure how to take that comment, but I doubt they write down just anything to fill up a page.

I did read the comments in the instructions provided, and will say again, this does not make any sense.

I'm open for a discussion on why the refrigerant is needed to defrost the evaporator coil though. Opinions have nothing to do with it. If it's based on sound principles then it works.

And as for the P-trap not being required, the evaporator is already trapped by the suction line riser going up.

The text pertaining to defrost sounds like it was intended for hot gas defrost, not electric.

Look who wrote it an EVAPORATOR co. they will put what They think is best for the EVAPORATORS they sell.
Now a compressor co. will say pump down w/ a 20deg supper heat @ the compressor what They think is best for the compressor they sell.
txv co. will say supper heat is taking @ the evap. what They think is best for the txv they sell.
what to do: cpr are only needed when you did not size the unit right in the first place.
pump downs are to stop slugging and stop the liquid from migrating to the condenser when its colder out side then in .

what to do: ask all the co. this the evpap the txv and compressor take to then .

[frank]

pump downs are to stop the liquid from migrating to the condenser when its colder out side then in

How does that work then??

[US Iceman]

txv co. will say super heat is taking @ the evap. what They think is best for the txv they sell.
what to do:

Superheat does not have much to do with the TXV. The TXV controls the superheat at the evaporator outlet, but there also different locations for superheat as far as a compressor is concerned.

If you have long uninsulated suction lines you could have perfect evaporator superheat and have too much suction superheat at the compressor. So, you have to judge the installation requirements.

cpr are only needed when you did not size the unit right in the first place.

Since these valves essentially provide the same function as a MOP valve, would you say a MOP valve is being used because the system is not designed correctly?

pump downs are to stop slugging and stop the liquid from migrating to the condenser when its colder out side then in .

Refrigerant can still migrate to the condenser even if a pump down is used.

I'm not throwing rocks, but I have worked for several manufacturers (which I would recommend for anyone who has the opportunity). This type of experience provides exposure to a lot of real world conditions and installation types. Sure, the manufacturers specify what they want (as they should).

After reading the reply of 750 valve I can see how the coil manufacturer is stipulating the requirements to ensure a prompt defrost, but then, you can see from his reply how they modify the installation to achieve the results they want.

His reply is interesting because it provides some details that apparently differ from the manufacturers, so you can see the real world applications sometimes override the theoretical world also.
As I mentioned before, I was willing to have an open discussion on this and 750 Valve provided some interesting input.

[tonto]

temprite,

I to have read this on paper work for buffalo coils and also have read it in an airah magazine i think it was as well.. I used to use a cpr valve on a freezer system, bur got recommended by an engineer at actrol to use a mop tx valve instead cause you are buying a tx valve anyway.. What are views on using mop tx valves??? I find the buffalo coils to have quite an aggresive defrost and never have problems with defrost anyway... I have tried pump down and not to pump down and havent seen much difference on the buffalo coils. What does everyone else think????

[Temprite]

I am waiting for a call back from the manufacturer.

I will try them again tomorrow.

[dsp]

Hi Temprite,
Kirby Evaporators are the same. They recomend to Not pump down thier evaps. for the same reason. Haven't heard back from Actrol parts either.

[adam]

we run a set up similar to 750's. we use a LLSV, which is set for shut down(as opposed to pump down) to prevent liquid migration. the pressure/temp of the small amount of refrigerant left in the coil helps to defrost the evaporator quicker. we use an MOP tx valve to prevent flood back on startup.

i was talking to an engineer at actrol and he said bitzer have told him it's o.k not to use a cpr with their low temp compressors. not sure about other manufacturers though.

[750 Valve]

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. Although granted CPR's freak a few people out - I've seen CPR's used in strange situations... like to limit HP on startup after defrost... try to figure that one out - its called making up for poor engineering!!!

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!) its just that like anything thats been lab tested - its never exactly the same in the multitude of applications and engineering variables that can go into a sytem. Sometimes you need to find what works for you.... if its pumping down the evap then so be it - just don't whinge to the mfg if you get coils icing over time at their specified defrost duration.

I'd be interested to know who that engineer at actrol is that says bitzer say its sweet to omit cpr's on their LT comps - this comes down to system design - maybe bitzer know the said engineer and are confident he sizes sytems generously, however there are guys out there pushing the limits and still engineering for lower ambients to save costs and win jobs, even an MOP won't save these systems - they take forever to pulldown after defrost for the obvious reasons, thats why that engineer shouldn't be making a blanket statement like that, if it was correct bitzer would publish the info which I bet my bottom dollar they don't.

[adam]

^ 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.