Compact heat exchanger

A shell-and-tube heat exchanger is used as an ammonia condenser with ammonia vapor entering the shell at 50°C as a saturated vapor. Water enters the single-pass tube arrangement at 20°C and the total heat transfer required is 200 kW. The overall heat-transfer coefficient is estimated from Table I0-I as 1000 W/m2 °C. Determine the area to achieve a heat exchanger effectiveness of 60 percent with an exit water temperature of 40°C. What percent reduction in heal transfer would result if the water flow is reduced in half while keeping the heat exchanger area and V the same  

The mass flow can be calculated from the heat transfer with 

Because this is a condenser the water is the minimum fluid and 

The value of NTU is obtained from the last entry of Table 10-4, with e = 0.6  

When the flow rate is reduced in half the new value of NTU is 

Matrix types, wire screen, sphere bed, corrugated sheets 

In design, the general approach is to obtain the term (M) for each stream, sum these for all the cold and all the hot streams and determine an overall value of (hA) given by  

In designing a unit, each side of the exchanger is independently tailored to the duty required, and the exchanger effectiveness (discussed in Section 9.9.4) can range from 2-5 per cent to values in excess of 98 per cent without fundamental design or construciion problems arising. Countercurrent, co-current and cross-flow contacting can be employed individually or in combination. 

A note of caution on the use of photo-etched channels has been offered by Ramshaw - who points out that the system is attractive in principle provided that severe practical problems such as fouling are not encountered. With laminar flow in matrices with a mean plate spacing of 0.3-1 mm, volumetric heat transfer coefficients of 7 MW/m- K have been obtained with modest pressure drops. Such values compare with 0.2 MW/m K for shell and tube exchangers and 1.2 MW/m K for plate heat exchangers. 

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Noncircular tubes are often used in various compact heat exchangers and the Reynolds number in these tubes is of interest. For noncircular tubes such as square, rectangular, eUiptic, and triangular tubes, the so-called hydrauhc diameter, defined as... 

A. K. Singhal, L. W. Keeton, A. K. Majundar, and T. Mukerjee,M Improved Mathematical Formulation for the Computations of Flow Distributions in Manifolds for Compact Heat Exchangers, paper presented at The ASME Winter Annual Meeting, Anaheim, Calif., 1986, p. 105. 

For rectangular ducts Kays and Clark (Stanford Univ, Dept. Mech. Eng. Tech. Rep. 14, Aug. 6, 1953) published relationships for headng and cooling of air in rectangular ducts of various aspect rados. For most noncircular ducts Eqs. (5-39) and (5-40) may be used if the equivalent diameter (= 4 X free area/wetted perimeter) is used as the characteristic length. See also Kays and London, Compact Heat Exchangers, 3d ed., McGraw-Hill, New York, 1984. 

Noncircular Ducts Equations (5-50 ) and (5-50/ ) may be employed for noncircular ducts by using the equivalent diameter D = 4 X free area per wetted perimeter. Kays and London (Compact Heat Exchangers, 3rd ed., McGraw-HiU, New York, 1984) give charts for various noncircular duels encountered in compact heat exchangers. 

For extended surfaces, which include fins mounted perpendicularly to the tubes or spiral-wound fins, pin fins, plate fins, and so on, friction data for the specific surface involved should be used. For details, see Kays and London (Compact Heat Exchangers, 2d ed., McGraw-HiU, New York, 1964). If specific data are unavailable, the correlation by Gunter and Shaw (Trans. ASME, 67, 643-660 [1945]) may be used as an approximation. 

KAVS, w. M. and London, a. l.. Compact Heat Exchangers, McGraw-Hill, 1964... 

Compact heat exchangers Advantages of compact units... 

Colburn equation, flat plates 434 Colorimetric methcxls for humidity determination 759 Comings, E. W, 745, 786 Compact heat exchangers 550 Compressed air for pumping 358 Compressibility factor 34, 35,161 Compressible fluids 30, 48... 

The definition of mini-channels and micro-channels has not been clearly and strictly established in the literature although many related studies have been done. For example, for compact heat exchangers, Mehendale et al. (1999) gave a relatively... 

Shah RK, London AL (1978) Laminar flow forced convection in ducts a source book for compact heat exchanger analytical data. Advances in Heat Transfer, suppl 1. Academic, New York Sher 1, Hetsroni G (2002) An analytical model for nucleate pool boiling with surfactant additives. Int J Multiphase Flow 28 699-706... 

Heatric (UK) Compact heat exchangers for offshore facilities, hydrogen production 4 tons per year [1]... 

C.H. (1997) Compact heat exchangers as chemical reactors for process intensification (PI). Process Intensification in Practice, BHR Group Conference Series, Publication No. 28, pp. 175-189. 

CHART Industries (2009) Compact Heat Exchange Reactors. Available on mviv.cha rt-i nd.com /app ec reactortech.cfin (accessed March 2009). 

Compact heat exchangers MicroChannel heat exchangers Rotor/stator mixers Rotating packed-bed Centrifugal adsorber... 

A detailed characterization of micro mixing and reaction performance (combined mixing and heat transfer) for various small-scale compact heat exchanger chemical reactors has been reported [27]. The superior performance, i.e. the process intensification, of these devices is evidenced and the devices themselves are benchmarked to each other. 

Interview with Worz/BASF in a special on heat exchangers giving expert opinion on compact heat exchangers, feasibility and problems of large-scale implementation of micro reactors measuring tool for process optimization exotic status. scale-up unit-construction kit industrial implementation in 5 years [216],... 

Kays, W. M., and A. L. London, 1958, Compact Heat Exchangers, National Press, Palo Alto, CA. (3) Keeys, R. K. F., J. C. Ralph, and D. N. Roberts, 1971, Post Burnout Heat Transfer in High Pressure Steam Water Mixtures in a Tube with Cosine Heat Flux Distribution, UK Rep. AERE-R-6411, AEA, Harwell, England. (5)... 

Certain sectors of industry are seeking to use lightweight and corrosion-resistant compact heat exchangers for condensation as well as for convection duties. This requirement is of particular interest in the aviation, automobile and domestic heating and ventilation industries. Also, recent interest in the concept of mobile chemical plants necessitates the use of lightweight compact heat exchangers. Such plants may well have an important role to play in the future of processing as they provide flexibility, improved inherent safety... 

BURNS, J. R. and JACHUCK, R. J. J. App. Thermal Eng. 21 (2001) 495-510. Condensation studies using cross-corrugated polymer film compact heat exchanger. 

W. M. Kays and A. L. London, Compact Heat Exchangers, McGraw-Hill Book Co., Inc., New York, 1964. 

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