refrigerant to water heat exchanger

[knighty]

http://cgi.ebay.co.uk/20-Plate-Brazed-Heat-Exchanger-CE-certified-45-bar_W0QQitemZ110309974439QQcmdZViewItemQQptZUK_Home_Garden_Hearing_Cooling_Air?hash=item19aefca1a7&_trksid=p3286.c0.m14&_trkparms=65%3A12%7C66%3A4%7C39%3A1%7C72%3A1683%7C240%3A1318%7C301%3A1%7C293%3A1%7C294%3A200

will this be ok to use on the liquid line of a freezer room at -20'C - running 404A ?


new bigger room being built, used for storage and freezing of product... we always have a lot of problems on the very hot summer days...

to combat this, I'd like to have heat exchangers installed on the liquid lines, water cooled, with temp sensors so the water only flows when it's needed

that way the system won't be taken out of balance by adding the heat exchanger....

if the liquid line should be at 25'C, the temp sensor could be set for say 27'C.... on a hot day when the condenser is struggling to dump all the heat and the liquid lines is hotter than designed the temp sensor will trip the pump/water valve and cool the liquid line down.... once it's back down to 25 the pump/valve trip off


is this idea ok ?

and are those heat exchangers above ok ?

[tmm]

Knighty, I presume from your enquiry that the condenser is clean and the superheat setting is correct?
The problem with the plate exchanger is the risk of failure resulting in water/refrigerant contamination.
I would always try and avoid the potential and offer an alternative scheme. Some years ago I was called to a supermarket with a condensing unit which was undersized. Whilst a new unit was on order the problem facing me was to reinstate the unit after an HP switch operation. Prior to resetting the system I attended the gardening section of the store and 'acquired' the garden sprays which people normally use for the lawn. These were linked to a supply on the roof and placed beneath the condenser. The system was started and operated throughout the remainder of the summer until the fitment on the new unit. Mk 2 used a solenoid valve to open the water feed linked to a thermocouple placed on the condenser header.
If you remember your steam tables this type of cooling is called adibatic and this takes the short form of 'ADBATS' for the equipment which is now marketed, not by me I might add, to overcome the problem.
Just a thought?
tmm

[desA]

The problem with the plate exchanger is the risk of failure resulting in water/refrigerant contamination.
I would always try and avoid the potential and offer an alternative scheme.

A very interesting comment, often raised in support of using double-walled heat-exchangers. Do you perhaps have any experiences with this kind of failure on plate HX's?

The better plate HX manufacturers will claim that this very rarely happens in practice, if correct operating procedures are used.

[littleyapper]

hoping not to cause hassle but if you are having problems on a warm day the plant (condensor) was not sized correctly... but any way i am a huge fan of heat exchange and should be used to reclaim heat where possible (not to compensate for undersized equip)there is a regulation en1717 that says you have to use double walled exchangers now to stop cross contaimination ... also if you do buy from ebay be carefull of pressure drop etc...

[knighty]

hiya guys, thanks for the input

I only linked to the ebay one as an example... because I don;t really know where to buy this kind of thing...

it's not the "warm" days which are a problem... it's those one off really hot days that screw us up... the meat in the freezers stays frozen... but the fresh stuff we put in there each day doesen't freeze... whgich causes us all sorts of problems...

the new room should be way over-specked... but at the same time... thats what we said about the ones we have now... as we get busier and busier freezers should be upgraded too... but they end up neglected....

could someone point me to where I could buy some good double walled heat exchangers ?

I would do with the "spray/mist over the condenncor" idear..... but I'm worried about corrocsion eating through it.... :(

[littleyapper]

if you are in the uk try rpw and mention a company called "dk" ... i dont work for rpw by the way but i do use these dk tanks in ireland with great success

[tmm]

A very interesting comment, often raised in support of using double-walled heat-exchangers. Do you perhaps have any experiences with this kind of failure on plate HX's?

The better plate HX manufacturers will claim that this very rarely happens in practice, if correct operating procedures are used.
desA, The immediate answer to your question is YES! As you are most probably aware you can specify the plates with/without tails for either connections- but typically the water will come extended. The 'advice' from the manufacturer is to part fill the plate with water when making the brazed connection(s). I have now attended 4 of these which either failed almost within a week or perhaps of greater concern at around six months to a year. I believe that the latter is as a result of heat distortion during the brazing process?
As most applications employe hermetic scrolls you can imagine the problems that resulted from the failures!
Perhaps not enough to form an opinions but when specifing these units I always order extended tails and avoid creating problems for myself!!

TMM

[desA]

Perhaps not enough to form an opinions but when specifing these units I always order extended tails and avoid creating problems for myself!!


Good points.

Agreed. I always have the plate HX manufacturer supply & install & pressure-test all brazed connections. I simply will not take a chance with incorrect brazing procedures, on dissimilar & different gauge materials.

The incremental on-cost to doing this far outweighs the potential failures in service.

So, in the cases mentioned, the failures could perhaps more correctly be attributed to to incorrect fitment & brazing practices, rather than plate HX mechanical design, or operational failures perse.

[tmm]

Good points.

Agreed. I always have the plate HX manufacturer supply & install & pressure-test all brazed connections. I simply will not take a chance with incorrect brazing procedures, on dissimilar & different gauge materials.

The incremental on-cost to doing this far outweighs the potential failures in service.

So, in the cases mentioned, the failures could perhaps more correctly be attributed to to incorrect fitment & brazing practices, rather than plate HX mechanical design, or operational failures perse.
desA, agree with the points raised on the matter of installs rather than problems with the plate but I've had one, with R134a, which I believe failed internally. With a compressor its obviously quite easy to cut open and verify the cause of the failure- not quite as easy with the plate!! I suspect that these, with the current uptake in requests for designs of secondary systems from my clients, will become much more common and perhaps more failures will provided the basis for on-going analysis?
Just as a final thought given the Pressure Vessel Regs this application must be getting close to actuation of the PRV if fitted?

tmm

[tmm]

Knighty, I spent a little time thinking on this and admit that I was lead by your idea to use the exchanger. So without wishing to complicate matters any more why not uprate the condenser fan motors? In the first instance you may be able to obtain new blades with an increased angle of attack, hence inproved airflow across the coil, without changing anything else but you need to check the motor rating as the extra load could cause motor failure or tripping if not sized correctly. As an option replace the motors and feed them via a head pressure controller?
As you mentioned that the freezer room is being upgraded this might be the easiest option.
Hope it helps. tmm

[Gary]

Why use it only when needed? Why not subcool all the time?

[knighty]

tmm: I wasn't sure how much better fan blades / fan motors would help on a hot day ?

summer is comming... and I do have 3 or 4 very large / fast extractor fans lying around... I guess I could put one of those infront of the condensor and try to duct the air through it ? -the problem is I won;t really be able to test to see how well it works... it's tricky to test fot the lack of a problem, if you know what I mean ?


gary: everything works fine most of the time... we only have problems on those really hot days.... also, I was worried about throwing the system out of balance... also... (maybe most inportantly) even using the mains fead water and exiting it into the hot water headder tank... I'm not sure how much we can use... might well end up dumping it down the drain :(

(as long as it's only for the really hot days it should be ok)

[Goober]

Knighty, I'm with tmm's original response with regard to using garden sprinkler on condenser, although I'd use something a bit more sophistcated than a garden sprinkler. We had a similar problem with several supermarkets here that had not been upgraded for a while and during a particularly hot summer, (and with a bit of scaremongering, re the world getting warmer) we installed permanant water "atomising type" spray systems under all the affected condensers, our plumber installed them, and we controlled the water valve off the plant control system. Worked like a dream, and the atomising type spray thingys used a lot less water than garden sprinklers. Not sure where you're coming from worrying about corosion of condensers, sure are they not out in all weather anyway?