There are two basic types of thermal design problems, namely, rating and sizing. In a rating problem, the geometry and size of the heat exchanger are fully specified. Entering flow rates and fluid temperatures are known. The job is to calculate the thermal effectiveness (heat transferred) and pressure drop of each stream. This is a quite straightforward problem, with one exception. Because the exit stream temperatures are not known, the average temperatures at which the fluid properties are evaluated are not known.

In a sizing problem, the heat exchange requirement is specified and the designer must calculate the heat exchanger size. Normally, pressure drop limits are given for each fluid stream. Since the entering flow rates, temperatures, and pressures are given, and the heat duty (or leaving temperatures) is specified, the thermal effectiveness ε and NTU (number of transfer units) are directly calculable. A true sizing problem is considerably more complex than the rating problem. A number of decisions must be made prior to making the thermal performance calculations. These include the selection of the following.

1. Heat exchanger flow arrangement, e.g., counterflow, cross flow, etc.
   
   
2. Heat exchanger materials, as influenced by fluid temperatures and corrosion potential.
   
   
3. Fin geometry and fin thickness, as influenced by design pressure requirements.
   
   
4. The type of surface geometry and fin spacing and height. Fouling considerations influence the type of surface geometry and fin spacing that may be selected. Fin height is influenced by the desired fin efficiency.
   
   
5. Heat exchanger frontal area. This key decision establishes the Reynolds number for each flow stream. The pressure drops are directly dependent on this decision. Use of high-performance surfaces (high j-value) will tend to require a larger frontal area and less core depth (and less core volume) for a specified pressure drop. If a small frontal area is preferred, surfaces having a high value of j /f will support this need. However, such a selection will likely require greater core volume, and heat exchanger material.

For a sizing calculation, the calculation result cannot be completed by a single, step-by-step rational decision path. The main complicating factor is the choice of available surface geometries and how this affects the limiting pressure drops. Since several candidates are likely, a number of heat exchanger designs that meet the specified thermal and pressure drop performance are possible. Identification of the optimum design requires considerably more effort. First, the designer must establish what variables are to be optimized, e.g., first cost, operating cost, or size. Design optimization is discussed by Shah and Sekulic (2003) and Palen et al. (1974).

Aside from the optimization question, the first crucial element of a design problem is selection of the surface geometries for which the heat exchanger size will be calculated. The second crucial element is selection of the flow frontal areas (or velocities) on which the pressure drops depend. Methods for selecting the fluid velocities are discussed in Section 306. Once the surface geometry and flow velocities have been specified, the thermal design calculation is relatively straightforward. The calculation steps are basically the same as for a rating problem.