Bypass heat exchanger

Count the control valves. The number of control valves available in a process equals the degrees of freedom for control. Most of the valves will be allocated to the basic control features, as production rate, control of inventories (gas and liquid), product quality control, as well as safety and environmental constraints. The remaining valves can be used to enhance steady state economics or improve dynamic controllability. Additional manipulations can be obtained by bypassing heat exchangers or some separators. 

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

PAFC systems are commercially available from the ONSI Corporation as 200-kW stationary power sources operating on natural gas. The stack cross sec tion is 1 m- (10.8 ft"). It is about 2.5 m (8.2 ft) tall and rated for a 40,000-h life. It is cooled with water/steam in a closed loop with secondary heat exchangers. The photograph of a unit is shown in Fig. 27-66. These systems are intended for on-site power and heat generation for hospitals, hotels, and small businesses. Another apphcation, however, is as dispersed 5- to 10-MW power plants in metropolitan areas. Such units would be located at elec tric utihty distribution centers, bypassing the high-voltage transmission system. The market entiy price of the system is 3000/kW. As production volumes increase, the price is projec ted to dechne to 1000 to 1500/kW. 

The control valve in the delivery line to the furnace was also closed. Unfortunately, this valve was bypassed by the line through the heat exchanger. In the heat of the moment, no one remembered to close the valve in the bypass line. In addition, the check (nonreturn) valve did not hold. The return flow of oil from the furnace was stopped by closing a hand valve next to the furnace, which was about 30 m from the fire. Afterward, another EIV was installed in the pump delivery line. 

Higher overall heat transfer coefficients are obtained with the plate heat exchanger compared with a tubular for a similar loss of pressure because the shell side of a tubular exchanger is basically a poor design from a thermal point of view. Considerable pressure drop is used without much benefit in heat transfer efficiency. This is due to the turbulence in the separated region at the rear of the tube. Additionally, large areas of tubes even in a well-designed tubular unit are partially bypassed by liquid and low heat transfer areas are thus created. 

It is shown in Section 9.9.5 that, with the existence of various bypass and leakage streams in practical heat exchangers, the flow patterns of the shell-side fluid, as shown in Figure 9.79, are complex in the extreme and far removed from the idealised cross-flow situation discussed in Section 9.4.4. One simple way of using the equations for cross-flow presented in Section 9.4.4, however, is to multiply the shell-side coefficient obtained from these equations by the factor 0.6 in order to obtain at least an estimate of the shell-side coefficient in a practical situation. The pioneering work of Kern(28) and DoNOHUE(lll who used correlations based on the total stream flow and empirical methods to allow for the performance of real exchangers compared with that for cross-flow over ideal tube banks, went much further and. 

Simpler optimization problems exist in which the process models represent flow through a single pipe, flow in parallel pipes, compressors, heat exchangers, and so on. Other flow optimization problems occur in chemical reactors, for which various types of process models have been proposed for the flow behavior, including well-mixed tanks, tanks with dead space and bypassing, plug flow vessels, dispersion models, and so on. This subject is treated in Chapter 14. 

Unavoidable loss of gas is compensated via a feed valve for supplying virgin nitrogen into the circulation pipe. The exhaust gas of the process has to be bypassed for purification. After the separation of dust by a filter, the gas is heated to 400 °C for the catalytic combustion of the side products. The gas is then cooled down, and the excess oxygen is catalytically converted to water by using hydrogen. For economic reasons, the gas flow will recover the heat via a heat exchanger and then be cooled down by a gas cooler. 

The connecting tubing should be as short and narrow as possible. The volume of a 20 cm x 0.5 mm i.d. tube is 39.3 p. That of a 20 cm x 0.25 mm i.d. tube is 9.8 /A, and that of a 20 cm x 0.125 mm i.d. tube is 2.5 [A. Some detectors are equipped with a heat exchanger that consists of a metal block containing a capillary tube. The volume of this tube also affects the theoretical plate number. If highly sensitive operation is not required, the heat exchanger can be removed or bypassed. 

Water is pumped from an atmospheric tank into a vessel at 50 psig through a heat exchanger. There is a bypass around the heat exchanger. The pump has a flat curve. The heat exchanger pressure drop is 30 psi with 200 gpm of flow through it Size the pump and the two control valves so that ... 

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