Pressure drop is a major constraint in the thermal design of shell and tube heat exchangers. A thermal design of a shell and tube heat exchanger is meaningful only when it is optimum and the extent of the optimality is constrained by the pressure drop.
Optimization of thermal design requires maximization of overall heat transfer coefficient and/or effective mean temperature difference (EMTD) to minimize the heat transfer area subject to the constraints, pressure drop being the major one. Other constraints may be flow-induced vibration, space limitation, etc. Overall heat transfer coefficient can be maximized by maximizing shell side and tube side flow velocities, which, in turn, is governed by the allowable pressure drop as higher velocity means higher pressure drop. The maximization of EMTD is achieved by a pure counter-current flow. If there are two passes on the tube side or shell side, a pure counter current flow requires two passes on the other side, but allowable pressure drop in that side may not allow the same.
Why measuring the pressure drop is crucial for heat exchangers?
The pressure drop should be managed in such a way that the calculated pressure drop is within and as close as possible to the allowable pressure drop. On the other hand, if the pressure drop is surplus during thermal design, the calculated pressure drop should be increased as close as possible to the allowable pressure drop.
Managing the pressure drop
Pressure drop limiting situation:
When the pressure drop is a limiting factor, the objective of the thermal design of the exchanger should be to reduce the calculated pressure drop. Ultimately, the calculated pressure drop should be as close as possible to the allowable pressure drop without exceeding it. Shell side pressure drop can be reduced in the following ways:
- Changing the baffle type from segmental to double segmental. This reduces the shell side flow velocity and, thereby, lowers the shell side pressure drop
- Increasing the baffle spacing. This increases the crossflow area and, hence, decreases cross-flow velocity resulting in decreased pressure drop. However, this can be done in a limited way only as maximum unsupported tube span should be limited to TEMA recommended value
- Increasing the baffle cut. An increase in baffle cut increases the window flow area and, therefore, reduces window velocity, thereby, reducing the pressure drop. This, too, has limited impact as only window pressure drop is affected and the contribution of window pressure drop is generally small to total pressure drop. Moreover, it can be varied in a limited range only and generally does not exceed 35 percent of the shell inside diameter
- Increasing shell diameter (thereby reducing tube length). This increases the flow area, reduces flow velocity, and, hence, lowers pressure drop. However, this lowers tube side heat transfer coefficient as well because of lower tube velocity. Moreover, higher shell diameter means higher shell thickness and a larger number of tubes, hence, higher material cost
- Using no-tubes-in-window (NTIW) baffles. With this type of baffle, baffle spacing can be increased as much as required by providing a sufficient number of intermediate support plates, thus, limiting the unsupported tube span within the TEMA requirement. However, a large fraction of shell volume remains unoccupied by the tubes with this type of baffle which results in lower heat transfer area to volume ratio and, hence, higher cost
- Changing the type of shell. Other aspects remaining the same, TEMA J shell yields less pressure drop than TEMA E shell as the shell-side flow is divided and flow velocity becomes half
- Further pressure drop reduction can be achieved in the TEMA H shell as it has a double split flow and reduced velocity. Pressure drop is least in TEMA X shell which has cross-flow, largest flow area, and least velocity
- Increasing the tube pitch ratio. An increase in tube pitch ratio reduces the cross-flow velocity and, thereby, reduces the pressure drop. However, tube pitch ratio is generally 1.25, 1.33, or 1.50 and, therefore, can be varied in a limited way only
- Increasing the nozzle size. If the pressure drop across the nozzles is excessive relative to the total pressure drop, the nozzle size can be increased reasonably to lower the pressure drop
Using the shells in parallel. Multiple shells can be used in parallel so that total shell-side flow is split and flow velocity is reduced. Consequently, the pressure drop is reduced. However, it increases the cost due to the increase in the number of shells, tube sheets, channels/ bonnets, nozzles, flanges, etc. Tube side pressure drop can be lowered in the following ways:
- Increasing the shell diameter. Increasing the shell diameter increases the tube flow area due to the increased number of tubes and, thereby, reduces tube flow velocity and, hence, reduces tube side pressure drop. Further, it also means reduced tube length which, too, leads to reduced pressure drop
- Increasing tube diameter. An increase in tube diameter reduces tube velocity and, thereby, reduces pressure drop. However, tube diameters are standardized and standard outside diameters are limited as given in table 1. Further, tube outer diameter more than 1.0 inch is generally not desirable as higher tube diameter means higher shell diameter to accommodate the required number of tubes due to increased tube pitch which, in turn, means higher cost
- Increasing the nozzle size. If nozzles are too small in diameter, their diameter can be increased reasonably to lower the pressure drop
- Using the shells in parallel. Multiple shells can be used in parallel so that total tube side flow is split and flow velocity is reduced. Consequently, the pressure drop is reduced. However, it increases the cost due to the reasons as mentioned for shell side pressure drop
Pressure drop surplus situation:
When the pressure drop is surplus, the objective of the thermal design of the exchanger should be to utilize the available pressure drop fully, i.e. increase the pressure drop till it reaches as near as possible the allowable pressure drop.
Shell side pressure drop can be increased in the following ways:
- Reducing the shell diameter. This increases shell-side flow velocity and, thereby, increases the shell side pressure drop. However, the minimum shell inside diameter is generally limited to 6 inches. Further, it also leads to increased tube side pressure drop. Therefore, it can be done only if the tube side pressure drop also allows
- Reducing the baffle spacing. Reduction in baffle spacing increases the cross-flow velocity and, therefore, increases the pressure drop. However, minimum baffle spacing is generally limited to one-fifth of the shell inside diameter or 2″, whichever is larger. Moreover, reduction in baffle spacing leads to an increased fraction of shell-side flow by-passing the tube bundle, thereby, lowering the heat transfer. Therefore, it can be done in a limited way only
- Changing the shell type. If the pressure drop cannot be fully utilized with the TEMA E shell, one should use the TEMA F shell with a vertical cut segmental baffle. This increases shell-side flow velocity and, hence, pressure drop. However, for a pure counter-current flow, the TEMA F shell should be combined with two tube passes, which means increased tube side pressure drop, too
- Using shells in series. If the pressure drop cannot be fully utilized with a single shell, shells in series can be used. However, this increases tube side pressure drop as well and, therefore, can be done only when tube side pressure drop allows so. Further, it results in increased cost due to increased shells, channels/bonnets, tube sheets, nozzles, flanges, etc
Tube side pressure drop can be increased in the following ways:
- An increasing number of tube passes. If the tube side pressure drop in an exchanger with one tube pass cannot be fully utilized, the pressure drop can be increased by increasing the number of tube passes which increases the tube flow velocity and, hence, pressure drop. However, with the TEMA E shell, the use of 2 or more tube passes lowers the EMTD which means a higher required heat transfer area. Therefore, the combined impact of increased heat transfer coefficient and reduced EMTD on the required heat transfer area should be evaluated
- Reducing tube diameter. Reduction in tube diameter increases pressure drop as it increases tube velocity due to decreased flow area. However, lower tube diameter can create difficulty in mechanical cleaning of the inside of tubes. It may also create a flow-induced vibration problem
- Reducing the shell diameter. Reduction in shell diameter increases tube velocity due to reduced flow area and, hence, increases tube side pressure drop. However, it also increases the shell side pressure drop. Therefore, this option can be exercised only when the shell side pressure drop also allows. Further, the minimum shell inside diameter is generally limited to 6 inches
- Using shells in series. If the pressure drop remains unutilized with one shell, shells in series can be used. However, this also increases shell side pressure drop, and, therefore, can be done only when shell side pressure drop allows so. Further, it has cost impacts as described above for shell side pressure drop
