Re: Evaporator rating
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:
Originally Posted by otrotabi
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.
Q' = U.A.F.dTlm
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]
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).
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.
Take a look at the literature reference provided above. All the best in your journey.
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
Last edited by desA; 28-04-2012 at 07:43 AM.
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