Cocurrent and countercurrent flow heat exchanger

Based on their configuration, heat exchangers can be labelled as tubular, shell and tube or plate heat exchangers. Heat exchangers are typically classified according to flow arrangement and type of construction. The simplest heat exchanger is one for which the hot and cold fluids move in the same or opposite directions. In the counter current flow arrangement of Figure 1, the fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends. In the parallel-flow arrangement of Figure 1, the hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end. Figure 1: Concentric tubes heat exchangers, parallel flow and counter flow In this exercise, the effect of changing the direction of fluid flow on the heat transfer and temperature distribution in a Plate heat exchanger will be evaluated. Counter current and co-current behaviors are depicted in Figures 2 and 3 below. Temperature Efficiencies & Temperature Profiles For the flows depicted in Figure 1, we seek to determine the heat lost to the surroundings and the overall efficiency of the heat exchangers. The power emitted from the hot stream can be obtained from the following relation 𝑸̇𝒆 = 𝑽̇𝒉𝒐𝒕 𝝆𝒉𝒐𝒕 𝒄𝒑 (𝑻𝟏 − 𝑻𝟐 ) 1 Similarly, the power absorbed by the cold stream obtained from 𝑸̇𝒂 = 𝑽̇𝒄𝒐𝒍𝒅 𝝆𝒄𝒐𝒍𝒅 𝒄𝒑 (𝑻𝟒 − 𝑻𝟑 ) 2 where the temperatures (T) in Equations 1 and 2 are the inlet and outlet water temperatures for the hot and cold fluids and 𝑽̇, 𝝆 and cp the volumetric flow rates, densities and specific heats of the hot and cold fluid. Assuming water to be the fluid in question, Tables 1 and 2 can be used to obtain values of densities and specific heats. Also, since water densities and specific heats are dependent upon temperature, for either the hot or cold streams, the mean temperature Tmean = 𝑻𝒊𝒏𝒍𝒆𝒕 + 𝑻𝒐𝒖𝒕𝒍𝒆𝒕 3 𝟐 can be used to obtain these values from Tables 1 and 2 below. Figure 2: Counter Current Operation Figure 3: Co-Current Operation The heat power lost (or gained) can be obtained as |𝑸̇𝒆 | − |𝑸̇𝒂 | 4 and the overall efficiency given by 𝜼𝑷 = 𝑸̇ 𝒂 𝑸̇ 𝒆 x 100 5 If the heat exchanger is well insulated, then both Qa and Qe should be equal. However, these may differ due to heat losses or gains to/from the environment. If the average cold fluid temperature is above the ambient air temperature, then heat will be lost to the surroundings. On the other hand, if the average cold temperature is below the ambient temperature then heat will be gained. Thus, in extreme cases, this can result in an apparent thermal efficiency greater than 100%. Temperature efficiencies of the hot and cold streams and the overall efficiency of the exchanger can be obtained from Figures 2 and 3. Overall Heat Transfer Coefficient The logarithm mean temperature difference (LMTD) is defined as 𝑳𝑴𝑻𝑫 = ∆𝑻𝒎𝒂𝒙 − ∆𝑻𝒎𝒊𝒏 ∆𝑻 𝐥𝐧 ( ∆𝑻𝒎𝒂𝒙 ) 𝒎𝒊𝒏 𝟔 where ΔTmax and ΔTmin are as indicated in Figures 1a and b below. Note that since the temperature measurement points are not fixed on the heat exchanger, the relation for ΔTmax and ΔTmin are not the same for both counter-current and co-current flow. The flow through the plate heat exchanger is not consistently either co- current or countercurrent, due to the nature of plate arrangements and the flow passages. Thus, a correction coefficient must be applied to the overall heat transfer coefficient.