Heat exchangers counter-flow

Figure 3.11 Photograph of the multi-plate stack reactor, originally designed as a counter-flow heat exchanger this type of reactor was also used for periodic operation [13].

P 68] No detailed experimental protocol was given [61, 62,142,143]. Two reactant streams, the solution of the reactant in hexane and concentrated sulfuric acid, were fed separately in a specially designed micro reactor by pumping action. There, a bilayer was formed initially, potentially decomposed to a dispersion, and led to rapid mass transfer between the phases. From this point, temperature was controlled by counter-flow heat exchange between the reaction channel and surrounding heat-transfer channel. The reaction was typically carried out at temperatures from 0 to 50 °C and using residence times of only a few seconds. If needed, a delay loop of... 

Figure 7.2 Operation of parallel-flow and counter-flow heat exchangers and their associated temperature profiles.

Usually, the counter-flow heat exchanger is smaller than the parallel-flow heat exchanger because the LMTD of the former exchanger is larger than that of the latter exchanger when the inlet and outlet temperatures of the hot fluid and the inlet and outlet temperatures of the cold fluid are identical. The following example illustrates this comparison. 

A counter-flow heat exchanger heats water at 101 kPa from saturated liquid state to saturated vapor state. The temperature of the heating flue gas (air) entering and leaving are 1800°C and 1200°C at 101 kPa. Find (A) the LMTD, rate of water flow, and heat transfer based on a unit mass of heating flue gas, and (B) the LMTD, rate of helium flow, and heat transfer for a parallel-flow heat exchanger under these identical operating conditions. 

Bejan, A., "The Concept of Irreversibility in Heat Exchanger Design Counter-flow Heat Exchangers for Gas-to-Gas Applications," Trans. ASME J. Heat Transfer, 99, 374 (1977). 

Determine UA for using the applicable equations for counter-flow heat exchangers. 

Counter-Flow Heat Exchangers 624 Multipass and Cross-Hovr Heat Exchangers Use of a Correction Factor 625... 

SOLUTION Hot oil is cooled by water in a douhle-tube counter-flow heat exchanger. The overall heal transfer coefficient is to be determined. Assqmptions 1 The thermal resistance of the inner tube is negligible since the tube rhaterial is highly conductive and its thickness is negligible. 2 Both the oil and water flow are fully developed. 3 Properties of the oil and water are constant. 

The variation of temperatures of hot and cold fluids in a counter-flow heat exchanger is given in Fig. 11-16. Note that the hot and cold fluids enter the lieat exchanger from opposite ends, and the outlet temperature of the cold fluid in this case may execed the outlet temperature of the hot fluid. In the limiting case, the cold fluid will be heated to the inlet temperature of the hot fluid. However, the outlet temperature of the cold fluid can never exceed the inlet temperature of the hot fluid, since this would be a violation of the second law of thermodynamics. 

The relation already given for the log mean temperature difference is developed using a parallel-flow heat exchanger, but we can show by repealing the analysis for a counter-flow heat exchanger that is also applicable to counterflow heat exchangers. But this lime, AT, and AT2 are expressed as shown in Fig. 11-15. 

For specified inlet and outlet temperatures, the log mean lemperaiuie difference for a counter-flow heat exchanger is always greater than that for a parallel-flow heat exchanger. That is, AT, cf > AT, pp, and thus a smaller surface area (and thus a smaller heal exchanger) is needed to achieve a specified heat transfer rate in a counter-flow heat exchanger. Therefore, it is common practice to use counter-flow arrangements in heal exchangers. 

A condenser or a boiler can be considered to be either a parallel- or counter-flow heat exchanger since both approaches give the same result. 

The log mean temperature difference Ar) , relation developed earlier is limited to parallel-flow and counter-flow heat exchangers only. Similar relations arc also developed for cross-flow and niiillipass shell-and-tiibe heat exchangers, but the resulting expressions are too complicated because of the complex flow conditions. 

The correction factor is less than unity for a cross-flow and multipass shcll-and-lube heat exchanger. That is, F s I. The limiting value of F - 1 corresponds to the counter-flow heat exchanger. Thus, the correction factor Ffor a heat exchanger is a measure of deviation of the AT,from the corresponding values for the counter-flow case. 

Anelysis The schematic of the condenser is given in Fig. 11-19. The condenser can be treated as a counter flow heat exchanger since the temperature of one of the fluids (the steam) remains constant. 

EXAMPLE 11 4 Healing Water in a Counter-Flow Heat Exchanger... 

Discussion The inner tube of this counter-flow heat exchanger and thus the heat exchanger itself) needs to be over 100 m long lo achieve the desired heat transfer, which is impractical. In cases like this, we need to use a plate heat exchanger or a multipass shell-and-tube heat exchanger with multiple passes of tube bundles. 

That is, the existing hot-water stream has the potential to supply heat at a rate of 67.9 kJ/s to the incoming cold water. This value would be approached in a counter-flow heat exchanger with a very large heat transfer surface area. A beat exchanger of reasonable size and cost can capture 75 percent of this heat... 

C In the heat transfer relation Q = UA ATi for a heal exchanger, what is AT, called How is it calculated for a parallel-flow and counter flow heat exchanger ... 

C Can the outlet temperature of the cold fluid in a heal exchanger be higher than the outlet temperature of the hot fluid in a parallel-flow heat exchanger How about in a counter-flow heat exchanger Explain. 

A double-pipe counter-flow heat exchanger is to cool ethylene glycol (cp = 2560 J/kg °C) flowing at a rate of 3.5 kg/s from 80 C to 40°C by water (c, = 4180 J/kg C) that enters at 20°C and leaves at 55 C. The overall heat transfer coefficient based on the inner surface area of the lube is 250 V/m "C. Determine (o) the rate of heat transfer, (A) the mass flow rate of water, and (c) the heat transfer surface area on the inner side of the tube. 

Water (Cp = 4180 J/kg °C) enters the 2.5-cm-inlemal-diameter lube of a double-pipe counter-flow heat exchanger at 17°C at a rate of 3 kg/s. It is heated by steam condensing ai 120°C (hfg = 2203 kJ/kg) in the shell. If the overall heat transfer coefficient of the heat exchanger is 1500 W/m °C, determine the length of the lube required in order to heat the water lo 80°C. 

C Consider two double-pipe counter-flow heat exchangers that are identical except that one is twice as long as the other one. Which heal exchanger is more likely to have a higher effectiveness ... 

C Under what conditions can a counter-flow heat exchanger have an effectiveness of one What would your answer be for a paral lei -flow heat exchanger ... 

Consider an oil-to-oil double-pipe heat exchanger whose flow arrangement is not known. The teinpersiure measurements indicate that the cold oil enters at 20°C and leaves at 55°C, while the hot oil enters at 80"C and leaves at 45 C. Do you think this is a parallel-flow or counter-flow heat exchanger Why Assuming the ma.ss flow rales of both fluids to be identical, delermine the effectiveness of this heal exchanger. 

Consider a water-to-waler counter-flow heat exchanger with these specifications. Hot water enters at 95°C while cold water enters at 20°C, The exit temperature of hot... 

In practice, we encounter three types of heat exchangers, classified according to the flow of one fluid relative to that of the other. In a parallel-flow heat exchanger both fluids flow in the same direction in a counter-flow heat exchanger the fluids flow in opposite directions and in a cross-flow heat exchanger the fluids flow at right angles... 

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