Evaporator rating

[otrotabi]

Hi everyone,

I have a question regarding evaporators (air evaporators). As far as I know, they are all governed by the equation Q = k A Dt, where k depends on a number of factors including geometry, air speed, refrigerant speed, but not absolute temperature, only the difference between refrigerant temperature and and air temperature.

However I have seen some tables supplied by manufacturers where there is a correction factor to be considered when working at different evaporation temperatures, for example, considering that the evaporator performance is given at tev = -7 °C, you need to multiply this performance by 0.93 if the system will work at tev = -20 °C, even when Dt = 8° C in both cases.

I have not been able to find technical references to this subject, it is obvious that mass flow will be less at lower temperatures and so refrigerant speed will decrease also, but I would like to know the real story behind these tables.

Look forward to your comments

[aramis]

K may not DIRECTLY depend on absolute temperature but it does indirectly while Q depends directly on mass flow which depends on density which depends on absolute temperature.

K depends on absolute temperature because the film heat transfer coefficients depend on absolute temperature and in many materials (like metals) conduction coefficient depends on absolute temperature too. In normal application's Dt you can assume conduction coefficients constant but not film coefficients.

Therefore Q depends on suction temperature. (The correct formula is Q=m k A Dt)

[aramis]

By the way, welcome to RE forums.

As for references, any heat transfer handbook will do, like this one:

http://www.thermalfluidscentral.org/e-books/book-intro.php?b=37

Or you may search the Refrigeration Books Forum.

[otrotabi]

Thanks aramis for your kind answer. I understand film coefficients will be different depending on refrigerant mass flow or refrigerant speed, whichever you choose. Do you happen to know if there should be a further correction depending on air conditions (RH %) or fins are assumed to be perfecly clean, even when working at low temperatures ?

[aramis]

Do you happen to know if there should be a further correction depending on air conditions (RH %)

If the documents are certified by any trustable authority, you (generally) shouldn’t need further correction. If they are not certified then it depends on you if you trust the supplier. In any case the documentation should state if it requires more correction factors.

For example serious European manufacturers should be Eurovent certified: http://www.eurovent-association.eu/recommendation/documents_en_000049.html


…or fins are assumed to be perfecly clean, even when working at low temperatures?

Eurovent has strict definitions for the minimum time and maximum fluctuation of your measurements and the ambient conditions for the test, so the coils should be at real working conditions (i.e. frosted). Read section PG07. ¡Aunque te advierto che, es aburrido!

[otrotabi]

ahhh, bueno. No había leído tu perfil. Gracias por la respuesta, seguramente los fabricantes locales han tomado esa tabla a la cual hago referencia de algún fabricante europeo o algún libro, pero uno nunca sabe. Saludos. José.

[aramis]

Yeah that's called "copy and paste".

The problem with uncertified data is that you probably will need a subjective (unpublished) reliability or confidence factor.

[desA]

Hi Otrotabi,

Welcome to the wonderful world of heat-exchanger design. A number of your questions can be answered by looking at a few Heat-Transfer text books. The chapter on heat-exchangers should answer a lot of your questions.

A very useful free download is found at : http://web.mit.edu/lienhard/www/ahtt.html

Let's start looking at your query in some level of detail.


Hi everyone,
I have a question regarding evaporators (air evaporators). As far as I know, they are all governed by the equation Q = k A Dt, where k depends on a number of factors including geometry, air speed, refrigerant speed, but not absolute temperature, only the difference between refrigerant temperature and and air temperature.

The governing equation for heat-exchanger design with sensible heat-transfer for both fluids, is of the form:


Q' = U.A.F.dTlm
where :
Q' = heat-transfer duty [W]
U = overall heat-transfer coefficient [W/m2.K]
F = cross-flow heat=transfer correction factor [-]
dTlm = log mean temperature difference [K]

Things get a little more complicated when phase-change fluids are involved. For these units, the designer generally takes a view (based on experience) for the de-superheating & sub-cooling (condenser) or superheat (evaporator), then selects the dTlm based on the phase change section of the unit.


Otrotabi:
However I have seen some tables supplied by manufacturers where there is a correction factor to be considered when working at different evaporation temperatures, for example, considering that the evaporator performance is given at tev = -7 °C, you need to multiply this performance by 0.93 if the system will work at tev = -20 °C, even when Dt = 8° C in both cases.

This correction factor is basically accounting for the changing dTlm value, with temperatures which are different to those for which the original heat-exchanger was designed/rated. The temperature effect on the overall heat-transfer coefficient (U) & cross-flow correction factor (F), is generally ignored for such a rough-cut re-rating. In some cases, a diligent designer will run a full set of design cases, which allow for all known variations with temperature, then plot these in a performance chart (quite rare, in my experience as it requires a lot of work).


Otrotabi:
I have not been able to find technical references to this subject, it is obvious that mass flow will be less at lower temperatures and so refrigerant speed will decrease also, but I would like to know the real story behind these tables.

Look forward to your comments

Take a look at the literature reference provided above. All the best in your journey. :)