> Higher > velocity means less time in the cooler which means less time for heat to > transfer. >
I'm not disagreeing on the comments about pressure drop. They are correct and my pressure drop model is simplistic. After all, this is a freebie program. I do disagree with the above statement though.
Residence time in a heat exchanger is not even considered by any design calculations. What is considered is Reynolds numbers and Prandl numbers and those have velocity terms in them. Higher velocities increase the film coefficients. The Reynolds numbers and Prandl terms are used to calculate the film coefficients and in that manner a kind of back door relationship to residence time is established.
>From a theory point of view, one would want to push the velocities as high as possible because that raises transfer coefficients. The down side is that pressure drop limits how much you can push the velocities.
Another problem with increasing residence time is that it forces the temperatures of the two fluids closer. At first glance it appears that you are transferring more heat with a closer approach temperature. It doesn't work that way. Heat transfer is governed by Log Mean Temperature Difference. It takes a temperature difference to make heat move. Close approaches don't transfer heat. Even worse, its a LOGARITHMIC function. Higher velocities move more mass flow. The higher mass flow decreases approach temperatures. The higher temperture difference drives more heat transfer.
About 20 years ago I designed preheat recovery exchangers for the liguid discharge of methanol distillation columns. I was preheating the liquid mixture of methanol, ethanol, and water so that less heat had to be added to the column to make everything boil using the heat left in the "slops" of the column. The original exchangers were huge units with very low velocities because the original designers of the plant wanted to make sure that they had plenty of time to transfer the heat from the "slops" and they didn't want them to plug. They didn't transfer heat very well and plugged up every week.
My designs used very high velocities. The velocity was so high that residence time was less than a half second. Everyone in the plant said that they wouldn't transfer heat because of no residence time. They also said that they would plug almost immediately because of the small tubes and 8 pass arrangements. The originals were either single pass or two pass. The debate went all the way to corporate and they eventually chose my designs.
To make a long story short, my designs averaged three times the heat transferred in a unit half the size and plugged about every other month.
I made some simplifications in the IC Designer program. Most of my design texts are family of curves based. I simply couldn't easily transfer that information into a coherrent equation for use in a computer program nor did I want to take the time to build tables and program an interpolation routine. I solved across a wide range of velocity, Reynolds numbers and Prandl numbers by hand. I was surprised that the film coefficient stayed relatively constant. By that I mean within +/- 20% of a median arrangement. After I thought about it I came to realize that air doesn't have the ability to alter its film coefficient as much as water. One reason is that the viscosity of air is relatively constant while water varies considerably. I decided to simplify a program that I was giving away for FREE by using fixed film coeficients with only a correction for water on one side.
My experience in the business of designing heat exchangers for a LIVING made and makes me beleive that my solutions are close enough for the purposes at hand.
I have designed power house blow down heat exchangers, condensers for purification systems (pressure drop is all important there), coolers for hydraulic systems, Heat recovery systems, condensate flash units, and air to water heater cores.
My favorite heat exchanger texts are Process Heat Transfer by Donald Kearns and the Kay and London book on Compact Heat Exchanger Design. Both are out of print now.
Pulling out the soap box, Generic statements are very difficult to prove true or false. Very few people will take the time to solve a heat transfer problem looking at extremes so as to see how sensitive the solutions are to changing parameters. I've done that.
UPDATE
Prior to Carlisle, I was working on an Excel spreadsheet that would analyze intercooler performance. The spreadsheet became so complex and slow that I had to rethink it and do it as a dedicated computer program.
The computer program is available for download on both the Merkurowners site and Ryan Matson's site.
ICDesigner is a Visual Basic program that works from your estimated volumetric efficiency, engine size, boost, level, and atmospheric conditions to compute the engine air intercooler exit temperature for a lot of commonly available and some not so common intercoolers. It will also allow you to user define an intercooler.
The program has the ability to store and recall engine files. Several are included in the zip program.
ICDesigner rates intercooler performance in what most people call "Heat Soaked" state. It's what I call Steady State performance. Intercooler discharge temperatures will normally be lower than the calculated level unless the engine is held at full boost for up to 45 seconds.
I have created an email ID on Merkurowners so that people can ask questions. That ID is ICDesigner@Merkurowners.org.
Questions on the inner workings of the program can only be explained using thermodynamic equations. Some knowledge base is necessary for that. I will make every effort to explain as simply as possible, but, remember that heat exchanger design is one of the most complex engineering problems in existance.
The program is a freebie. It is in a 3.7 MEG zip file. After unzipping, just run setup to install it. I will continue to refine it as time goes by. I don't expect to do much more with it for several months. I intend to make it capable of handling compound charge air cooling as well as to revise its ability to compute heat transfer coefficients.
BUYER BEWARE:
This program is a theoretical model of a real process. I have
made every effort to instrument my car to verify it's accuracy.
It appears to be accurate on estimated discharge temperatures
within 5%. The program also generates horsepower and torque curves
based on YOUR estimate of the volumetric efficiency of YOUR engine.
I will not be held responsible if the estimated power curves are not correct. Nor will I be held responsible if you use this information to blow your engine up.
Enough said,
Enjoy,
Don Haulsee
