OCR Text |
Show 3. Packed columns 4. Cross-flow-tray columns 5. Pipeline contactors Generally, only the first three items mentioned above are used for heat transfer service. Item #4, cross-flow-tray columns, are generally too expensive, except for vacuum fractionation conditions where low pressure drop and heat and mass transfer considerations are critical. Item #5, pipeline contactors, are not considered suitable for the type of heat transfer required in flue gas condensation heat recovery. Therefore, the following discussion will be limited to the first three items mentioned above, namely baffle-tray columns, spray chambers, and packed columns. It is important to realize that in this type of equipment, there is basically a tradeoff between heat transfer performance and gas side pressure drop. Table 1 shows the relative heat transfer and pressure drop characteristics of the three devices. The spray chambers have low heat transfer performance per unit of volume and correspondingly low pressure drop requirements (~0.1 inches of water). For a majority of the small industrial boiler applications, these units have been placed on forced draft or balanced draft boilers without requiring any additional fan capacity with associated controls and increased operating costs. However, in some cases the breeching requirements were such that an additional fan and damper controls were required thereby partially defeating the low pressure drop features of the spray chamber unit. The baffle plate column represents the compromise between heat transfer and pressure drop. The heat transfer performance of the baffle plate is superior to a spray chamber device, but it has a correspondingly greater pressure drop. It would be anticipated that in some applications, additional fan capacity could be required in a baffle plate column but the general level of actual pressure drop within the column should be in the neighborhood of 0.5 to 1.0 inches of water. The packed tower has the highest heat transfer 9-10 KVB10-P-281 |