There are other ways to kill unwanted organisms (contaminants), such as using chemicals and filtration. However, using heat energy seems to be the best way to sterilize feed before entering to the reactor.
Here are topics that will be discussed in this presentation:
- How does a heat exchanger work?
- What does the performance of a heat exchanger depend on?
- what are some common problems with heat exchanger?
1. How does a heat exchanger work?
A heat exchanger utilizes the fact that heat transfer occurs when there is a difference in temperature.In a heat exchanger, there is a cold stream and a hot stream. The two streams are separated by a thin, solid wall. The wall must be thin and conductive in order for heat exchange to occur. Yet the wall must be strong enough to withstand any pressure by he fluid. Copper seems to be one of a common choice for construction.
Here is a simple flow diagram showing how heat transfers in a heat exchanger.
This flow arrangement is called co-current. If the direction of one of the stream is reversed, the arrangement is called counter-current flow.
Here are the temperature profiles along the heat exchanger. Note that the temperature profiles are different for co-current flow and for counter-current flow.
. 
The area between the curve is the heat transfer rate (Q). We can see that the heat transfer rate for counter-current flow is larger than the rate for co-current flow.
Counterflow heat exchanger provides more effective heat transfer. Most of the industrial heat exchangers are counter-current flow design.
Design of a heat exchanger varies with needs. Click on the names to see the design of the heat exchanger. These pictures are taken from the "Continuous Sterilization" exercise from RPI's Chemical Engineering department.
Image with permission
from Dan Nolan of Southern Heat Exchanger
Corp.
Plate-and-Frame Heat Exchanger
Cooling Coil Heat Exchanger: There
used to be a nice picture at this site for Owen's Publications. Now the
link
goes to their home page in case you would like to browse to try and
find it.
2. On what does the performance of a heat exchanger depend?
To maximize the performance of a heat exchanger means saving money, especially if the process is built for a long-term project. Here are some ways to improve the performance of a heat exchanger:- heat transfer area
- fluid flow velocity
- temperature gradient
Q=U*A*dTlm
where
- Q = Heat transfer rate between the fluids
- U = Overall heat transfer coefficient
- A = Heat transfer area
- dTlm = Log mean temperature difference of the system
Heat transfer area
As the equation shown above, the heat transfer area (or contact area) is directly proportional to the heat transfer rate. If the heat transfer area increases, heat transfer rate increases as well.A common way to increase heat transfer area is adding fins
to the surface. It is cheap to put fins to the heat transfer area but fins
also increase fouling, especially in bio-process.
Fluid flow rate
The importance of the fluid flow in a heat exchanger is that it changes the overall heat transfer coefficient, U. The data obtained from the heat transfer experiment shows that the velocity of the cooling fluid is directly proportional to the overall transfer coefficient. The following is a plot of 1/U vs. 1/V0.8 during one of the runs during the lab experiment.
As the cooling fluid velocity increases, the cooling fluid is able to dissipate heat more effectively. The data have shown that it is the case.
Although increasing flow velocity can give more effective heat transfer, it may not be a good idea in some bio-process. High velocity creates high shear stress in flow. Some proteins' or cells' structures are very delicate. They cannot withhold such force and they will be destroyed. The whole batch can be ruined.
Temperature gradient
Temperature gradient is certainly a important part of heat transfer. It is the driving force for heat transfer. If we can introduce fluids with greater temperature difference into the heat exchanger, the heat transfer rate (Q) will be greater. If we go back to the temperature profiles of the co-current and counterflow, we can see that the driving force is great for co-current at the beginning but decreases drastically as it moves along the heat exchanger. The counter-current flow provides relatively consistent driving force and therefore performs better than co-current flow.
3. What are some common problems for heat exchanger in bio-process?
Fouling in tubes
Fouling is always a major problem in mass or heat transfer. Materials are concentrated at the transfer surface and they decrease the flux flow. In bio-process, the problem of fouling is more apparent. Many bio-materials (such as proteins and bacteria) can easily stick to the wall and cause fouling.Cleaning the tubes can be a major task. There are companies specializing in cleaning tubes. This is one of the companies that specializes in this kind of problems:
Sites from around the world with interesting applications of heat transfer:
University of Cambridge: Department of Chemical EngineeringThe researches from Dr. Ian Wilson and Processor John Bridgewater on food are interesting.
Water Pasteurization Techniques: This article discusses different ways to treat drinking water
Well, this has been the presentation on heat exchanger.
Click here to go to the Continuous Sterilization presentation from the Chemical Engineering Department at Rensselaer Polytechnic Institute.
Click here to see other term projects done by students from RPI's Chemical Engineering Department.
Web site
for cooling towersAnother
nice site showing towers for atomic energy plants
Cooling towers
References:
- Incropera & Dewitt. Introduction to Heat Transfer. 2nd edition. J Wiley & Sons, New York, 1990.
- Data from senior lab experiments done on September 26th, 1995.
This presentation is by Wai Wan.
For any comments, please email to:
Wai (wanw@rpi.edu)