Countercurrent and cocurrent heat exchangers

The operating characteristic F eilNilCi) = 0, for a countercurrent heat exchanger is found by analysing the temperature distribution in both fluids. The results can easily be transferred for use with the practically less important case of a cocurrent exchanger. 

We will consider the temperature changes, shown in Fig. 1.31, in a countercurrent heat exchanger. The temperatures, l and i)2 depend on the 2 coordinate in the direction of flow of fluid 1. By applying the first law to a section of length d2 the rate of heat transfer, dQ, from fluid 1 to fluid 2 through the surface element d/1 is found to be 

Now dQ is eliminated by using the equation for overall heat transfer 

The temperatures = i(z) and 2 = 2(z) will not be calculated from the two differential equations, instead the variation in the difference between the temperature of the two fluids /d1 — d2 will be determined. By subtracting (1.111) from (1.110) and dividing by (/d1 — 2) it follows that 

If Q 1, then takes on the character of an efficiency. The normalised temperature change of the fluid with the smaller heat capacity flow is known as the efficiency or effectiveness of the heat exchanger. With an enlargement of the heat transfer area A the temperature difference between the two fluids can be made as small as desired, but only at one end of the countercurrent exchanger. Only for 

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One optimum requires a uniformly distributed entropy production rate in a heat exchanger, mixer, or separator. Consider the example of countercurrent and cocurrent heat exchangers shown in Figure 4.11. Temperature profiles... 

Example 4.11 Energy dissipation in countercurrent and cocurrent heat exchangers The two most commonly used heat exchangers are countercurrent and cocurrent at steady-state flow conditions as shown in Figure 4.17. Estimate the energy dissipated from these heat exchangers if the surroundings are at 290 K. Consider the data below ... 

One optimum requires a uniformly distributed entropy production rate in a heat exchanger, mixer, or separator. Consider the example of countercurrent and cocurrent heat exchangers shown in Figure 4.4. Temperature profiles show that the driving force AP or 1/AP is more uniformly distributed in the counter-current than in the cocurrent flow operation. This is the basic thermodynamic reason why a countercurrent is better than a cocurrent operation. The duty of the exchangers depends on the flow rate and Met and outlet temperatures T and T2 of cold streams. The duty is the amount of heat transferred from the hot fluid to cold... 

Example 4.12 Energy dissipation in countercurrent and cocurrent heat exchangers... 

Figure 10 shows the differential heat exchange between a gas and a solids stream flowing countercurrently and cocurrently. The efficiency of the heat transfer equipment is to represented by the number of heat transfer stages... 

Figure 4.11. Heat exchangers with countercurrent and cocurrent operations.

Applying Eqs. (4.187) and (4.189) for cocurrent and countercurrent operations, we find Cocurrent heat exchanger I ... 

This ratio shows that the rate of energy dissipated in the cocurrent heat exchanger is almost twice the dissipation in the countercurrent heat exchanger. Although the heat exchanged between the hot and cold streams is the same, the countercurrent operation is thermodynamically more efficient. 

It was seen from the discussion of heat exchangers that the fluid streams are not strictly countercurrent. Baffles on the shell side induce crossflow, and in a two-tube-pass heat exchanger both countercurrent and cocurrent flow occur. To account for deviations from countercurrent flow, the logarithmic-mean teri5)erature difference is multiplied by a correction factor, F. Thus,... 

In this series reaction pathway, the desired species is the aldehyde. Since both reactions are exothermic (second reaction is highly exothermic), the reactor is operated nonisothermally. The reactor is a shell-and-tube heat exchanger consisting of 2500 tubes of 1 inch diameter. Should the heat exchanger be operated in a cocurrent or countercurrent fashion in order to provide a greater stabilization against thermal runaway ... 

A shell and tube heat exchanger is to be used to cool 100kgs of 98% sulphuric acid from 60°C to 40°C. Cooling water is available at 10°C and a flowrate of 50 kg s . The overall heat transfer coefficient is 500 W m " K . Determine the required surface area for (a) countercurrent and (b) cocurrent flow. Assume that the heat transfer coefficient is the same in both cases. (Heat capacities water, 4200Jkg K 98% sulphuric acid, 1500Jkg K" .)... 

Different flow arrangements exist for heat exchangers, namely, cross flow, countercurrent, and cocurrent flow. The main disadvantages of the cross-flow design are uneven temperature distributions, which also deteriorate the gas composition... 

In the first pass, both the hot and cold fluids flow in cocurrent flow through the heat exchanger, whereas in the second pass, the cold fluid now flows countercurrent to the hot shellside fluid. Half the heat exchange area is therefore in cocurrent flow and half in countercurrent flow. 

For G/S heat exchange, altogether eight cases may be differentiated, according to whether the operation is countercurrent or cocurrent, whether the solids are being heated or cooled, and whether the value of T is less or greater than unity. 

The practical heat-transfer coefficient is the sum of all the factors that contribute to reduce heat transfer, such as flow rate, cocurrent or countercurrent, type of metal, stagnant fluid film, and any fouling from scale, biofilm, or other deposits. The practical heat-transfer coefficient ((/practical) is, in reality, the thermal conductance of the heat exchanger. The higher the value, the more easily heat is transferred from the process fluid to the cooling water. Thermal conductance is the reciprocal of resistance (/ ), to heat flow ... 

In its simplest form, a heat exchanger may consist of two passages, with the cooling fluid in one passage and the warming fluid in the other. The flow direction of each of the fluids relative to one another may be countercurrent, cocurrent, or crossflow. 

Falling-film absorbers. These are usually vertical heat exchangers with the cooling medium in the shell and the absorption taking place in the tubes. The solvent flows downward, while the gas may enter either at the bottom (countercurrent flow) or at the top (cocurrent flow). 

Fig. 1.3. The effect of heat exchanger performance, NTU, and heat capacity ratio, h, on the normalized heat loss for heat-integrated processes in (a) countercurrent and (b) counter-cocurrent flow configuration.

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