Results 1 to 8 of 8
Thread: Evaporator rating
-
26-04-2012, 03:05 PM #1
Evaporator rating
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
-
26-04-2012, 08:52 PM #2
Re: Evaporator rating
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)Last edited by aramis; 26-04-2012 at 09:46 PM. Reason: Bolded text was edited after original post
-
26-04-2012, 08:54 PM #3
Re: Evaporator rating
By the way, welcome to RE forums.
As for references, any heat transfer handbook will do, like this one:
http://www.thermalfluidscentral.org/...intro.php?b=37
Or you may search the Refrigeration Books Forum.
-
26-04-2012, 10:43 PM #4
Re: Evaporator rating
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 ?
-
27-04-2012, 12:51 AM #5
Re: Evaporator rating
Do you happen to know if there should be a further correction depending on air conditions (RH %)
For example serious European manufacturers should be Eurovent certified: http://www.eurovent-association.eu/r...en_000049.html
…or fins are assumed to be perfecly clean, even when working at low temperatures?
-
27-04-2012, 11:47 AM #6
Re: Evaporator rating
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.
-
27-04-2012, 04:02 PM #7
Re: Evaporator rating
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.
-
28-04-2012, 07:32 AM #8
Re: Evaporator rating
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.
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]
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.
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 commentsLast edited by desA; 28-04-2012 at 07:43 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )