Heat Transfer and Exchangers

Chairman, Department of Chemical and Petroleum Refinery Engineering, School of Petroleum Sciences, University of Tulsa 

Although heat-transfer rates can be computed reasonable accuracy for clean or new pipe, the effect of dirty or corroded pipe surfaces cannot be satisfactorily estimated. With the greatest possible care, the accuracy with which the rate of heat transfer can be computed is probably not better than 5 per cent. 

There are three conimon methods of transferring heat, viz., radiation, conduction through solids, and conduction throu fluids. In practice a combination of two or even all of these methods may take place simultaneously. Radiation will be discussed in Chap. 18. 

Conduction is the most simple method of heat transfer. Newton s fundamental law of resistances and driving forces, if applied to conduction, takes the following form  

Q — Btu heat transferred per hr A = area of surface, sq ft Ti and Ti = temperatures at hot and cold surfaces 

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The LMTD, ie, logarithmic mean temperature difference, is an effective overall temperature difference between the two fluids for heat transfer and is a function of the terminal temperature differences at both ends of the heat exchanger. 

Eurther research on convective transport under low Reynolds number, quasicontinuum conditions is needed before the optimal design of such a micro heat exchanger is possible. The cooling heat exchanger is usually thermally linked to a relatively massive substrate. The effects of this linkage need to be explored and accurate methods of predicting the heat-transfer and pressure-drop performance need to be developed. 

U = coefficient of heat transfer and W,w = flow rate through external exchanger of hot and cold fluids respectively. 

There are many text books that describe the fundamental heat transfer relationships, but few discuss the complicated shell side characteristics. On the shell side of a shell and tube heat exchanger, the fluid flows across the outside of the tubes in complex patterns. Baffles are utilized to direct the fluid through the tube bundle and are designed and strategically placed to optimize heat transfer and minimize pressure drop. 

Kim, N.-H, Yun, f.-H., and Webb, R. L. Heat transfer and friction correlations for wavy plate fin-and-tube heat exchangers. Journal of Heat Transfer 119 (1997) August, pp. 560-567,... 

Chapter 3 of Volume 1 discusses many of the basic properties of gas and methods presented for calculating them. Chapter 6 of Volume 1 contains a brief discussion of heat transfer and an equation to estimate the heat required to change the temperature of a liquid. This chapter discusses heat transfer theory in more detail. The concepts discussed in this chapter can be used to predict more accurately the required heat duty for oil treating, as well as to size heat exchangers for oil and water. 

Buildup of condensate in a heat exchanger can cause operating problems as well as water hammer. If the steam supply is controlled by a motor valve and the valve is not fully open, the steam pressure may be too low to expel the condensate, and its level will rise. This will reduce heat transfer, and ultimately the steam supply valve will open fully and expel the condensate. The cycle will then start again. This temperature cycling is bad for the heat exchanger and the plant and may be accompa-... 

The baffle cut determines the fluid velocity between the baffle and the shell wall, and the baffle spacing determines the parallel and cross-flow velocities that affect heat transfer and pressure drop. Often the shell side of an exchanger is subject to low-pressure drop limitations, and the baffle patterns must be arranged to meet these specified conditions and at the same time provide maximum effectiveness for heat transfer. The plate material used for these supports and baffles should not be too thin and is usually minimum thick-... 

Short, B. E., Heat Transfer and Pressure Drop in Heat Exchangers, Engineering Research Series No. 37, University of Texas, Austin, TX, pub. No. 4324, pp. 1-55 (1943). 

Test, P. L., AStudy of Heat Transfer and Pressure Drop Under Conditions of Laminar Plow in the Shell Side of Cross Baffled Heat Exchangers, Paper No. 57-HT-3, ASME-AlChE Heat Transfer Conference, ASME, New York, NY (1957). 

Typical velocities in plate heat exchangers for waterlike fluids in turbulent flow are 0.3-0.9 meters per second (m/s) but true velocities in certain regions will be higher by a factor of up to 4 due to the effect of the corrugations. All heat transfer and pressure drop relationships are, however, based on either a velocity calculated from the average plate gap or on the flow rate per passage. 

At times it may be desirable to chemically clean various parts of the boiler plant and its auxiliaries. The reason may be curative or preventative. With low-pressure boilers and heat-exchange equipment, cleaning is carried out to restore heat transfer, and, for boilers, to prevent overheating. With higher pressure boilers, the restoration of heat transfer is trivial and, whilst some overheating may be avoided, the main reason for chemical cleaning is to prevent corrosion. In nuclear plant, an additional reason for cleaning is to remove radioactive material and improve safety. 

NOTE Although superheater, reheater, and economizer heat exchangers all contain either steam or water in their respective tube bundles, they generally are not considered by boiler engineers and designers to be part of the steam-water circulation system s boiler surfaces. This distinction typically is reserved for the various tubes, connecting headers (manifolds), and drums that collectively provide the primary heat transfer and steam-generating facility. 

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