Evaporator Size
Evaporator Size
(OP)
How do I size an evaporator that is used to transfer 26000Btu/h from a 70%/30% MeOH/water solution FR 1.4gpm to R-22?
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Evaporator Size
Your requirements, other than tonnage, are opaque and I don't understand them.
At minimum you are going to need:
load (tonage)
coil face velocity
Entering air dry bulb and wet bulb
Leaving air dry bulb and wet bulb
coil material
system pressure parameters
properties of fluid to be cooled
coil tube velocities
and etc.........
RE: Evaporator Size
If it's a liquid, and 1.4 GPM is the flow rate then you will you will need:
the specific heat of the solution
Entering liquid temperature
Reativity of the solution to the heat exchanger materials
You have already pre-determined what your ΔT for the liquid is going to be based the 26,000 BTU/Hr and the flow rate. The supplier will probably need to know the operating pressures your working with and the available pump FT/hd capacity you have to work with.
I wonder how you came by the BTH/HR and the flow rate?, only because you didn't provide your entering liquid temperature and your required leaving liquid temperature. It seems you may have gotten the cart before the horse as they say.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Evaporator Size
One thing I left out they will need to know the density or weight per gal of the solution, sorry about that.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Evaporator Size
Is this correct?
RE: Evaporator Size
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Evaporator Size
RE: Evaporator Size
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Evaporator Size
1. LMTD
2. Effectiveness-NTU
LMTD is closest to what you've done, so let's go with that.
Q=U A F deltaTlm
U is the overall heat transfer coefficient
A is the area
F is a correction factor for flow arrangment (usually 0.6 to 0.85) which is read from a graph
deltaTlm is the log-mean temperature difference which is defined different ways for different flow arrangments...essentially it calculated the average difference in temperature between the two working fluids
now to complicate things more, because phase change is occuring, the heat exchanger has to be discretized into three segments (liq-liq, liq-2phase, liq-vapor)
each of these segments will have vastly different U-values which should be obtained through a nussult correlation. In the case of the 2-phase U-value, this evaluation is very complicated and hard to get accurate without some experimental data.
That being said, and since you don't appear to be familliar with HEXs, you should consult a HEX manufacturer which will be able to provide performance characteristics of their particular models. Care should still be taken as most OEMs overstate the performance by as much as 10 percent.
Still another worry is that the performance will degrade over time as fouling occurs. The heat transfer coefficients will decrease requiring the R-22 flow rate to be increased. Be sure to get the performance results in both new and fouled conditions.
RE: Evaporator Size
Thanks for the post although I'am not familiar with the terms, the concept makes sense and gives me a bone to chew on as it were.
Now where did I put that book on nussult correlations
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Evaporator Size