Counterflow units

An oil cooler is to operate with an inlet of 138°F and an oudet of 103°F, and the cooling water enters at 88°F and is to be allowed to rise to 98°F. What is the corrected MTD for this unit, if it is considered as (a) a concentric pipe counterflow unit, (b) a single-pass shell-two-pass tube unit, and (c) a parallel flow unit ... 

In 1974 the Atlantic City Electric Co. placed Unit 3 of its B L England Station into commercial operation. Condenser cooling for the unit is provided by circulating sea water in a closed-cycle, natural-draft system. The cooling tower selected for the site was a hyperbolic, counterflow unit. The thermal test instrumentation procedures and test data as well as drift measurement results are given. The paper indicates that the tower operates within design specifications for thermal performance and that it meets the environmental criteria regarding the drift. 

The permeate flow rate is not proportional to the overall pressure difference because it is made up of two components, and the oxygen and nitrogen fluxes are each pn rtional to partial pressure differences. When the feed pressure doubles the driving force for oxygen more than doubles, as shown by a sample calculation for a counterflow unit. Case I. Pj - 90 Ibj. /in. = 105 Ib /in. absolute At inlet. - 105 X 0.21 - 22.05 Ib /in. absolute... 

Figure 9-104. Spray-filled counterflow induced draft cooling tower. Used by permission of The Marley Cooling Tower, a United Dominion Co.

In parallel operation (sensible heat transfer), fluids A and B (Figure 10-30) flow in the same direction along the length of travel. They enter at the same general position in the exchanger, and their temperatures rise and fall respectively as they approach the outlet of the unit and as their temperatures approach each other as a limit. In this case the outlet temperature, tg, of fluid B, Figure 10-30, cannot exceed the outlet temperature, Tg, of fluid A, as was the case for counterflow. In general, parallel flow is not as efficient in the use of available surface area as counterflow. 

For an exchanger design, the unit requiring the smallest area will be the counterflow having the largest corrected LMTD = 25.5°F for this example. 

The plates are grouped into passes with each fluid being directed evenly between the paralleled passages in each pass. Whenever the thermal duty permits, it is desirable to use single-pass, counterflow (Figure 26.1) for an extremely efficient performance. Since the flow is purely counterflow, correction factors required on the LMTD approach unity. With all connections on the head of the unit, the follower is free for very quick access to cleaning and maintenance. The effect of multi-pass operation will be discussed later in this chapter. 

In the stripper/condenser, the gas is cooled by counterflowing condensate from the condenser. The temperature of the SO2 rich gas that leaves the condenser is used to control the amount of cooling medium that must be sent to the condenser. Condensate from the stripper is returned to the buffer mix tank. The SO2 rich gas, containing at least 90% SO2 with the remainder being water, is ready for transport to a process unit. In the refinery, this normally will be the SRU or acid plant, where it will be converted to elemental sulfur or sulfuric acid. The SO2 can also be compressed and transported if necessary. Eigure 16.9 shows a typical Belco LABSORB System. 

Air handling unit coils should have no more than eight fins per inch and be no more than six rows in depth to facilitate coil cleaning. Where more than four rows are required to obtain the cooling capacity, two coils should be placed in series. Spacing between the coils should be piped so that counterflow of air and water is achieved. Fins should be of the continuous, flat (noncorrugated) type. 

The kilns built for Houdriflow units, the first employing the gas lift principle, are cylindrical vessels 12 to 20 feet in diameter and contain one or two split-flow or counterflow burning zones. These regenerators are operated at pressures of from 3 to 10 pounds per square inch gage. The increase in pressure effects a further saving in burning volume, as... 

Air paths may be crossflow or counterflow. Fan placement can be ahead of the fill section (forced draft) or behind it (induced draft). Manufacturers use different criteria in sizing units. Figure 4.18 reviews one tower manufacturer s approach. Specific design details should be obtained from the tower manufacturer. 

A variation of the normal sieve tray is the Turbogrid tray developed by the Shell Development Company. The Turbogrid tray uses slots instead of circular holes and operates as a counterflow plate unit with no downcomers. A typical tray consists of a flat grid of parallel slots. The slots can be stamped perforations in a flat metal sheet or the open spaces between parallel bars. 

Calculate the time required to cool the batch described in the preceding example if the external heat exchanger is a 1-2 multipass unit rather than counterflow. 

Solvent extraction. The press cake emerging from a screw press still retains 3 to 15 percent of residual oil. More complete extraction is done by solvent extraction of the residues obtained from mechanical pressing. The greater efficiency obtained in the solvent extraction process encouraged the industry for direct application to oilseeds. In the United States and Europe, continuous extractor units are used in which fresh seed flakes are added continuously and are subjected to a counterflow of solvent by which intimate contact is achieved between the seeds and solvent. The common solvent for edible oil is commercial hexane or heptane, commonly known as petroleum ethers, boiling in the range of 146 to 156°F (63.3 to 68.9°C). After extraction, maximum solvent recovery is necessary for economical operation. The solvent is recovered by distillation and is reused. The extraction oil is mixed with prepress oil for refining. The extracted meals contain less than 1 percent of residual oil. 

In this application a mixed bed is widely used to remove or polish the ion residuals remaining in demineralized water after prior stage-wise coflow or counterflow demineralization. The anion cation resin volume ratio often differs from 1 1 for polishing mixed beds depending upon the anticipated loading of cations (sodium) and anions ( silica ). Since no hardness cations are present, only sodium, precipitation reactions are absent and a well designed unit with clean resins... 

The broad overall principles of vessel construction remain the same as for coflow units except for one crucial, and essential, exception. The counterflow mode of operation means that a conventional fixed bed will be unstable during regeneration or service depending upon whether employing upflow regeneration or upflow service respectively. Therefore all counterflow designs incorporate a means whereby the resin bed remains stable and packed at all times. Several hold down techniques may be encountered in installed counterflow plants as illustrated in Figure 10.4. 

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