View Full Version : Matching Evaporator with Condensing Unit

11-07-2001, 10:49 PM
I have a 2 HP Le Unite Condensing Unit which Ill be installing into a freezer unit, running on refrigerant R404 with a cabinet temperature of minus 18 Celcius.
I need to match the condensing unit with an evaporator. Normally I refer to the capacities listed in the manuals from my refrigeration parts wholesaler and use this as a guideline. My calculations tell me that on a TFH2480ZBR condensing unit evaporating at -30C the capacity quoted is 1628 watts.
How do I select an appropriate evaporator which will match this duty.
I need advice from my more learned colleagues what procedures, methods, calculations they would use.

12-07-2001, 06:30 PM

Normally most of us get paid for this kind of work.

You could select from first principles or get hold of the Searle Catalogue, I only reccomend them as they are the best, yes I used to work there...

I am assuming forced air evaporator (cooler) not exotica...

I know it's only a small unit but your concerns should be in no particular order environment e.g corrosion risk for material of construction, noise, defrost method and control, drain route and length, size and location including orientation, access for motor replacment. The market has a wide range of fan formats so air throw and volume are big issues. Really you will need to know cabinet size packing density and load cycles as all this matters. Please don't do a plonk plonk selection rating to rating + 10% as we used to make a fortune replacing bent, buckled and burnt evaporators selected without care.

Don't trust any non Eurovent supplier and use TD not TDM it's just a way of making poor coolers look good (discuss....)

13-07-2001, 08:39 PM
Thanks Derek, any idea where I can get hold of a current Searle catalogue ?

Another beauty of this forum is that we can pose questions and get advice for free..... I think...

14-07-2001, 12:42 AM
Check this out...

Low Temp: 1 hp = 4000 btu/h (so 2 hp ~ 8000 btu/h evaporator)
Med Temp: 1 hp = 8000 btu/h
High Temp: 1hp = 12000 btu/h

Now mind you this is just a good rule of thumb.

You could convert Watts to Btu's and know exactly what you need, but I suspect you have left some information out.... heat of rejection versus capacity perhaps?

15-07-2001, 08:00 PM
Thanks, Subzero. That is a fine rule of thumb regarding the capacity of a compressor at different operating suction pressures. It is coincidental that tonnage and horsepower are so close together with air conditioning applications.

Somewhere in the back of my head I had a rule of thirds that follows your shortcut.
How do I select an appropriate evaporator which will match this duty.
I need advice from my more learned colleagues what procedures, methods, calculations they
would use.

An evaporator will always match its duty. Dumb answer, I know.

Let's begin with the absurd. Argumentum absurdum?

You have an evaporator the size of a cpu processor with a fan on it. You connect it to a 20 hp compressor with R404A. This may not last long, but I would expect the computer chip will get really cold.. really, really cold. Perhaps minus 100 deg F Maybe even 200 deg F. The component of a compressor unit and the component of an evaporator are always going to balance. Match duty, so to speak.

Wrap the equator with a 5/8 OD copper tube and put it on a 1/2 hp compressor unit and it will also match duty, although the refrigeration effect will be difficult to discern.

In both cases, the compressor will fail.

Now, going back to reality.

Most blower coils are rated at what the manufacturer tells us what their capacity is at 10 deg F TD. The TD refers to the Temperature Difference between the saturated suction temperature and the temperature of the air passing through the coil. Let's not get fancy and worry whether this is log mean TD and such. It is simply air in versus saturated suction temperature.

If you double the TD, you also double the capacity of the blower coil. Neat thing about most blower coils (or unit coolers, as they sometimes show up in literature) is that they have their capacity in their nomenclature.

Most often, at least in the USA, you will see it in the last 3 numbers. So if you see a 120 at the end of a model, that means it provides 12,000 Btu/hr at 10 deg TD.

At a 15 dg F TD this coil can accomplish 18,000 Btu/hr. Nifty, if you think about it. Basic heat transfer.

If the coil can get colder, it can do more work. But to make the coil colder, the compressor has to do more work. Toss in condensing surface and you have a wonderful seesaw wherein the compressor can be smaller if the heat exchange components get larger. In the US, SEER ratings of packaged A/C units are simply that: Larger heat exchange surfaces... smaller TD's... higher suction pressures, lower discharge pressures... thus less energy consumption.

Most everybody sizes coils at the 10 deg TD rating. Most everybody, thus oversizes the coils. In a prep area, you can have a 20 to 30 deg TD, for example. Thus a coil rated at 12,000 btu/hr is now a coil capable of 36,000 btu/hr. Most freezers operate within a 15 deg F TD. -20 suction for a -5 room temperature, thus you generally find more than adequate evaporators in walk-ins sized according to their nomenclature.

It is, of course, not this simple. Matching duty must take into consideration air flow, fin spacing, and many other things. I think my flawed wisdom is not a bad start, however, as an answer to the original question. Non-metric as it is.

Great question Abe. Lot's to think about. What I love about a good question is how many other questions are standing in line, behind it. A lousy answer such as mine is simply a way to move the next good question toward the front.



15-07-2001, 11:11 PM

A lousy answer such as mine is simply a way to move the next good question toward the front.

Far from being a "lousy Answer" you have increased my knowledge and probably helped in some way make me understand the principles to make me a better engineer.

As I mentioned before, Im not looking for information in manuals where I choose willy nilly, or to ask the spares companies to work things out for me. I like to know myself that what I design or select is correct and ratified by me.

As an aside, the spares wholesalers always "oversize" components. This way they make more money and cover their backsides with the same token, lest they be accused of the system not operating correctly in extreme conditions.

The learning aspect of this forum is indeed an important one, and if members who know the ropes, or have expertise in certain , specific areas are willing to impart this, then surely the health of all us fridgetechs can only improve!

Thanks for your answer Dan.

15-07-2001, 11:13 PM
Hey, I forgot to mention Sub Zero for his "rule of thumb rule" and Steve for the Searle advice. Thanks guys

23-07-2001, 12:42 PM
Searle Manufacturing Co
Newgate Lane
PO14 1AR

01329 823344
01329 821242


23-07-2001, 04:52 PM
Good Site Derek... but I am not sure I agree whole heartedly with your Eurovent statement.
Don't trust any non Eurovent supplier and use TD not TDM it's just a way of making poor coolers look good (discuss....)

All refrigeration is system dependent... so if I size an evaporator and condensing unit for a "specific' load and sometime later the actual load changes my evaporator that was designed for just that particular load is now inefficient so we are right back to where we started... "the customer is paying more and the environment is suffering". Even if the load does not change, but the compressor wears or as the drier does its assigned duty... alot of things will change the operating parameters... and as Dan stated... the system will attempt to match the load. A poor sweat joint, marginal line sizing, oil returns all those and more can and will effect the efficiency of the entire system

Not knocking these products but it reminds me of how business' sell their product ... using scare tactics. Lets discuss... how is Eurovent superior to ANY other supplier?

You mentioned that there had been many oversized evaporators in the field with bent tubes etc... what has that to do with the manufacturer? It seems to me that it was constructed poorly from conception if that is the case... improperly sized by the installer or perhaps the customer had strict space requirements and a very specific loading requirement... medical or scientific purposes. I have seen customers use ice cream freezers to store uncovered alcoholic slushie drinks... uncovered fresh hot spaghetti sauce in a refrigerator! :eek:

Duties change... just a fact, but if you have an exact load that does not ever change then a coil which is guaranteed to meet an exact btu rating is perfect. Myself, I prefer to err on the side of caution, knowing that my customers needs change. I think plus 10% over actual load can be WAY to much capacity though... I think every system needs to be reviewed for capacity tolerances.

So, I agree, yet I dissagree with suggesting only Eurovent manufacturers are honest and provide quality product that can match a system requirements, save the customer money and the environment from abuse. Just my two cents...

24-07-2001, 01:26 PM
Nice points Dean

Starting from a sales selection certain companies use TDM ratings which when comparing what you think is like for like you are comparing TD with TDM and can result in undersized equipment.

A school of thought is that by undersizing all the 'experts' safety factors (10%) are reduced and the system is idealised. This leaves engineers presenting solutions that £ for £ (or is it $ for $) that can be financially undercut by 'legitimate' undersizing. That result is a system that say 340days a year is fine and 25 days become 'events'. If as a reputable engineer you deliver quality designs and implement good practice there should be no 'events' other than catastrophic failure which you have of course covered with suitable safety/back up systems.

Most of the damage I have encountered was poor overall selection including lack of drain, lack of drain 'U' trap, oversizing and under defrost resulting in heavy ice build up, no defrost, gas defrost with no standing drain time, using aluminium fin/copper tube in salt water fish storage, storage of hot liquids (ice tea) uncovered, total lack of dehumidification, no drain trace heater etc etc……. and of course sucking loose plasti wraps into the fan. Duty is not the only concern.

Eurovent is the safety factor to me just like automotive crash test data it allows informed choice from an independent (but industry supported) body. You may never need the information it provides on your chosen manufacturers selection data but when you do your lawyer will love you…..

Using straight line safety factors is for my current chosen bed of hot coals pressure testing. Using single safety factors in condenser/evaporator sizing is saying 'I will assume that I have forgotten something or the consultant is lying about the duty' . To get it right you need the cumulative effect: and that includes 'thinking' about if not including allowances for instance

Refrigerant Properties data say ± 5%
Compressor Performance based on say 5K Superheat and 10K subcooling
Pressure Drops for pipework including filters etc.
System Charge variations say ± 5 to 50gm
Effect of leak rate over time
Compressor wear over time
Effect of ambient variations
Evaporator / Condenser rating by calculation/experience/software say ± 5%
Instrumentation tolerance say ± 2K and ± 1% FSD plus PID /fuzzy equation selection.


Use qualified suppliers data.
Use a total installation approach
Base safety factors on real risk not 'guesstimates'

24-07-2001, 07:55 PM
The thing I dislike the most is "rushing"... it only costs alittle more to take the time and do it right the first time rather than "rushing through" and forgetting something that may cost much more.

Derek... explain exactly what you mean by TD and TDM? Maybe I am losing it here... temperature difference = TD but what is TDM... Mean Temperature Difference??? Product??? :confused:

25-07-2001, 02:33 AM
In my experience "safety factor" has always loomed as an accumulation of things to consider that point toward the reason for previously undersized equipment.

In the 70's, most US manufacurers were padding the equipment:

With non-ARI certified capacities on the unit coolers... cheating the Log mean differential as I recall, but I could be wrong about the details. But again, another 10 percent undersizing factor.

With 95% instead of 97% ASHRAE duty which to me seems like you have to accept that 5% of the time you can expect this equipment to not work as planned..... for condenser sizing.

With 10 Deg F subcooling advantage and 65 deg F suction capacity built into the compressor rating. Neither of which were necessarily realistic from one application to another.

With marginal heat exchange ratings boosted by condensing and evaporating TD's. All vectors pointing in the same direction.... including how cold something should be.... product versus discharge air/return temperatures.

Toss in all the plus or minus ten percents that are already built into the manufacturer's and electrical supplier's promises. Toss in the same ten percents that the machinists use. Toss in the additional ten percents that the customer is wrong about. You could go nuts. You hope there is some sort of statistical magic that makes all or some of these things disappear or at least Log Mean Average out.

So the pendulum can swing in either wrong direction. As it does and did and is doing and will certainly do again. The only sensible position for a pendulum is the part it passes through most quickly. The dead center. It pauses at the extremes, when it is changing its mind, so to speak.

It does no salesman nor manufacturer any good to lose an equipment sale. They are the Yin.

Engineers, technicians, etc. are the yang. Those who suffer with the goods sold.

No wonder we so often feel that we are hoisted by our yin-yangs.

I have to always laugh when I do an energy calculation with rigid math and in the back of my head realize that I could be plus or minus wrong by a factor of 100%.

I don't really know of any better way toward approaching truth, however.


25-07-2001, 08:57 AM
TD Air on to air off
TDM (METD) Log Mean difference estimating center of coil and effected by coil density.

Dossart does a really nice explaination in Principles of Refrigeration


Instrumentation man its the only truth (with approprirate tolerances of course)