Exchanger duty, estimation

Ethylene (C2H4, 30,000 Ib/hr) at 1,000 psia and 0 F is heated to 600 F and then fed to a plug-flow reactor. Estimate the heat exchanger duty necessary to accomplish the heating. 

One luuidred kmol per hour of subcooled liquid at 300 K and 3 bar is superheated to 500 K in a steady-flow heat exchanger. Estimate tlie exchanger duty (in kW) for one of tile following ... 

LMTD = log mean temperature difference, °F = number of rows N, = number of tubes per row APj, = air-side pressure drop, inch HjO Q = exchanger duty, Btu/hr R = number of tube rows T, = outlet process fluid temperature, °F T2 = inlet process fluid temperature, °F t, = estimated air outlet temperature, °F t, = calculated outlet air temperature, °F t2 = inlet air temperature, °F At = temperature difference, °F... 

Chapter 3 of Volume 1 discusses many of the basic properties of gas and methods presented for calculating them. Chapter 6 of Volume 1 contains a brief discussion of heat transfer and an equation to estimate the heat required to change the temperature of a liquid. This chapter discusses heat transfer theory in more detail. The concepts discussed in this chapter can be used to predict more accurately the required heat duty for oil treating, as well as to size heat exchangers for oil and water. 

The cooling duty can be provided by either making the draught tube an internal heat exchanger or with a heat exchanger in an external circulation loop. The mass transfer coefficient for external loop airlift Fermenter is estimated as 8... 

A process requires a flow of 4 kg/s of purified water at 340 K to be heated from 320 K by 8 kg/s of untreated water which can be available at 380, 370, 360 or 350 K. Estimate the heat transfer surfaces of one shell pass, two tube pass heat exchangers suitable for these duties. In all cases, the mean heat capacity of the water streams is 4.18 kJ/kg K and the overall coefficient of heat transfer is 1.5 kW/m2 K. 

If it can be assumed that world market forces will level out the prices of equipment, the UK price can be estimated from the US price by bringing the cost up to date using a suitable US price index, converting to pounds sterling at the current rate of exchange, and adding an allowance for freight and duty. 

Before equation 12.1 can be used to determine the heat transfer area required for a given duty, an estimate of the mean temperature difference A Tm must be made. This will normally be calculated from the terminal temperature differences the difference in the fluid temperatures at the inlet and outlet of the exchanger. The well-known logarithmic mean temperature difference (see Volume 1, Chapter 9) is only applicable to sensible heat transfer in true co-current or counter-current flow (linear temperature-enthalpy curves). For counter-current flow, Figure 12.18a, the logarithmic mean temperature is given by ... 

Air-cooled exchangers consist of banks of finned tubes over which air is blown or drawn by fans mounted below or above the tubes (forced or induced draft). Typical units are shown in Figure 12.68. Air-cooled exchangers are packaged units, and would normally be selected and specified in consultation with the manufacturers. Some typical overall coefficients are given in Table 12.1. These can be used to make an approximate estimate of the area required for a given duty. The equation for finned tubes given in Section 12.14 can also be used. 

Check if this exchanger is likely to be suitable for the thermal duty required, and estimate the pressure drop for each stream. 

Example 15.1 A hot stream is to be cooled from 300 to 100°C by exchange with a cold stream being heated from 60 to 200°C in a single unit. 1-2 shell-and-tube heat exchangers are to be used subject to IP =0.9. The duty for the exchanger is 3.5 MW and the overall heat transfer coefficient is estimated to be 100 W-m 2-K 1. Calculate ... 

Figure 19.3 Estimates of temperature-enthalpy profiles from existing exchanger heat duties and temperature.

All of the commercial simulators include models for heaters, coolers, heat exchangers, fired heaters, and air coolers. The models are easy to configure, and the only inputs that are usually required on the process side are the estimated pressure drop and either the outlet temperature or the duty. A good initial estimate of pressure drop is 0.3 to 0.7 bar (5 to lOpsi). 

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