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alireza
12-09-2003, 11:45 PM
As I understood, here are not many friends who interested to discuss about "Heat Transfer "Problems in AC&R systems.

As I have seen [at least in my country, Iran ] many of the system problems are because of poor heat exchanger design.

For example I have seen many new Water-cooled condensers which was working as Air-cooled condensers [you know what means this].
Some other same cases always force me to think about the Poor heat exchanger design as a significant factor of poor working a refrigeration system.

I think it is necessary having a discussion about the effects of the poor heat exchangers on a system and the ways of realizing this type of problems.

I will be thanks from who joins to this discussion

Best Regards to all friends

Alireza

Gary
13-09-2003, 05:53 PM
Refrigeration is by definition a series of heat transfers. We transfer heat from where it is not wanted to where it is not objectionable. This is done in several steps. In order to determine if a system is designed and/or operating properly, we need to examine how well the heat is being transferred at each transfer point. Heat transfer is EVERYTHING.

750 Valve
22-10-2003, 02:56 PM
would love to see a photo of a water cooled condenser used as an air cooled.... I'm trying to see it in my mind. Are they blowing air through the tubes!?!:confused:

DaBit
23-10-2003, 10:47 AM
The heat exchangers used can make or break things. Still it surprises me how little literature is available on the subject. With google I can find lots of information about general refrigeration, but almost nothing about designing efficient heat exchangers. Or more important: the factors determining heat exchanger performance (example: if I double water flow through a watercooled condenser, how much does it's capacity increase?).

And if you don't know what influences a heat exchanger's performance, how can you choose the right one? Most manufacturers give performance data, but only for a single condition.

I once tried to apply a computational fluid dynamics model (a fairly straightforward Navier-Stokes solver for incompressible fluids) to see what actually happens inside a heat exchanger, but even far above average PC's are totally incapable of running these models.

We overclockers all build our evaporators and heat exchangers to ideas we have, and ideas we copied from someone else. There is absolutely nothing more behind it than common sense, which I don't like.

herefishy
23-10-2003, 03:20 PM
Originally posted by 750 Valve
would love to see a photo of a water cooled condenser used as an air cooled.... I'm trying to see it in my mind. Are they blowing air through the tubes!?!:confused:

I'm sure that you have it backward, 750 :) . I have seen pics and descriptions of the refrigeration hobbyists, dissassembling a refrigerated appliance and placing the (previously) forced air evaporator into a bucket of water and glycol mixture in order to fashion a water chiller.

QQQ
26-10-2003, 07:39 PM
I assume that dear alireza speaking about /Water-cooled condensers which was working as Air-cooled condensers/ hinted at raised temperature of condensation, which occurs in result not correctly designed heat-exchanger.It is clear that for properly working heat-exchangers necessary to ensure a number of parameters as that - water treatment,flow rate,efficent tower operation, etc
Best Regards

rbartlett
26-10-2003, 09:03 PM
Originally posted by herefishy
I'm sure that you have it backward, 750 :) . I have seen pics and descriptions of the refrigeration hobbyists, dissassembling a refrigerated appliance and placing the (previously) forced air evaporator into a bucket of water and glycol mixture in order to fashion a water chiller.

i've worked on one..!

this was cooling some cnc wire cutting machines the evap was just a big coil block in a tank of water..the flow from the water flow/return mixed it up enough to give a reasonable heat transfer..

we eventually ripped it out and put in a big water chiller..

cheers

richard

DaBit
27-10-2003, 11:50 AM
For us refrigeration hobbyists putting an air cooled evaporator into liquid works fine. Usually there is only a small difference between secondary coolant temperature and SST, which indicates good heat transfer.

This indicates that the idea is a bit unconventional, but not bad. A clever and cheap solution to a problem.

RogGoetsch
31-10-2003, 08:18 AM
Several levels of problems with heat exchangers. Basic problem is economic: primary surface is expensive, secondary surface (fins, etc) is cheaper but increases fowling potential. So in a competitive world, they tend to be too small, less efficient (low bid gets the job because the customer does not figure operating costs into the equation).

Even if you don't bid for work and try to buy quality, manufacturers compete so products tend to be similar in efficiency.

Heat exchanger design has problems. Equations only get you in the ballpark and that's only if you are working with 5/8" dia and up (ASHRAE) . Below that, you're on your own.

Best way is build and test. Fluid boundary layer is the most significant factor. You minimize the boundary layer with maximum turbulence which requires more operating energy.

Manufacturers publish ratings in standard ranges because operation outside those ranges is usually expensive or impractical. (Law of diminishing returns.) Not to mention that the ratings are based not on calculations but on testing, and testing is also expensive.

But to do it yourself is a wonderful education. Thermistors and software interface to PC's are readily available.

You may remember in the movie The Matrix, where the operator watching the flow of numbers across the screen tells Neo he doesn't even see the numbers anymore, but sees the people they represent? Same thing if you set up a computer logging your data and watch the data stream.

If you change parameters and plot the output on a P-H diagram enough times, you get to a point where something similar happens.

nh3simman
21-03-2007, 09:51 PM
Still it surprises me how little literature is available on the subject..


Are you kidding? There are text books, journals, CD's and software by the drainpan full.

Google's not exactly the best place to look for technical references.