MAXIMIZING EMTD

EMTD can be maximized by selecting the proper exchanger configuration. This is particularly important in a

temperature-cross situation — that is, if the hot-fluid outlet temperature is lower than the cold-fluid outlet

temperature. If there is a temperature cross, EMTD is significantly reduced if there is deviation from pure

countercurrent flow. In such a situation, maximizing EMTD should be the key objective. EMTD can be maximized

by providing multiple shells in series or by selecting a configuration wherein shellside and tubeside flows approach

pure countercurrent flow [1]. Thus, the following guidelines can be applied, based on TEMA heat-exchanger type

standards:

i. For TEMA E shells, one tube pass with countercurrent flow yields the maximum EMTD

ii. For TEMA F shells, providing two tube passes with countercurrent flow maximizes the EMTD

iii. For TEMA G and H shells, EMTD can be maximized by providing an even number of tube passes, that is,

2, 4 or 6 passes

iv. For TEMA X shells, as the number of tube passes increases, the flow approaches pure countercurrent flow,

with the shellside and tubeside fluids entering from opposite ends. This means that if the shellside fluid

enters from the top, then the tubeside fluid should enter from the bottom, and vice versa. Thus, EMTD can

be maximized by increasing the number of tube passes to the extent possible (constrained by tubeside

allowable pressure drop) and making the two fluids enter from opposite ends

v. For TEMA J shells, pure countercurrent flow cannot be achieved, as the shellside flow is divided into two

streams with opposite flow directions. Hence, the maximum achievable EMTD is less than the maximum

achievable EMTD for E, F, G, H or X shells. Therefore, this type of shell is not preferred in situations

where maximizing EMTD is the objective. A J shell is generally used when minimizing the pressure drop is

the objective

However, sometimes other constraints may not allow a configuration with pure countercurrent flow, and force the provision of multiple shells in series. For example, if the tubeside fluid is cooling water, more than one tube pass may be required to ensure sufficient velocity to minimize fouling, but if there is a temperature cross, this can significantly reduce EMTD with one shell pass. Therefore, the use of F, G or H shells, or multiple shells in series, is desirable in such a situation to maximize EMTD. The shellside pressure drop may be too high to select the F shell configuration. In such a case, multiple E shells in series or the use of a G or H shell may be the preferred choice. Also, F, G and H shells may not be appropriate because of potential thermal leakage through longitudinal baffles (if uninsulated). If the reduction in EMTD due to thermal leakage can more than offset the EMTD gain due to pure countercurrent or near-countercurrent flow, again the preferred option is the use of multiple shells in series.