1、 STD-ASHRAE SRCH IJHVAC 3-3-ENGL 1997 W 759b50 0528034 L7b International Journal of Heating, Ventilating, Air-conditioning and Refrigerating Research Editor Raymond Cohen, Ph.D., P.E., Professor of Mechanical Engineering and Herrick Professor of Engineering, Purdue University, U.S.A. Associate Edito
2、rs Arthur E. Bergles, Ph.D., P.E., John A. Clark and Edward T. Crossan Professor of Engineering, Department of Mechanical Engineering, Aeronautical Engineering and Mechanics, Rensselaer Polytechnic Institute, U.S.A. Science, University of Oxford, United Kingdom Fire Research Laboratory, National Ins
3、titute of Standards and Technology, U.S.A. Arthur L. Dexter, D.Phil., C.Eng., Reader in Engineering Science, Department of Engineering David A. Didion, D.Eng., P.E., Leader, Thermal Machinery Group, Building and Ralph Goidman, Ph.D., Senior Consultant, Arthur D. Little, Inc., U.S.A. Hugo Hens, Dr.Ir
4、., Professor, Department of Civil Engineering, Laboratory of Building Physics, Katholieke Universiteit, Belgium Ken-khi Kirnura, Dr. Eng., Professor, Department of Architecture, Waseda University and President, Society of Heating, Air-conditioning and Sanitary Engineers of Japan, Japan Universitt Ha
5、nnover, Germany Universit de Lige, Belgium University of Wisconsin-Madison, U.S.A. University of California, Santa Barbara, U.S.A. Horst Kruse, Dr.-Ing., Professor, Institut fr Kltetechnik und Angewandte Wrmetechnik, Jean J. Lebrun, Ph.D., Professor, Laboratoire de Thermodynamique. John W. Mitchell,
6、 Ph.D., P.E., Professor, Mechanical Engineering, Dale E. Seborg, Ph.D., Professor, Chemical Engineering, Policy Committee Laurance S. Staples, Jr., chair Frank M. Coda Hans O. Spauschus Fritz W. Steimle W. Stephen Comstock Raymond Cohen Publisher W. Stephen Comstock Robert A. Parsons, Handbook Edito
7、r Scott A. Zeh, Publishing Services Manager Nancy F. Thysell, Typographer Jenny Otlet-Jakovljevic ASHRAE Staff Editorial Assistant 01997 by the American Society of Heating, Refrigerating and Air-Con- ditioning Engineers, Inc., 1791 Tullie Circle, Atlanta, Georgia 30329. Ali nghts reserved. Periodica
8、ls postage paid at Atlanta, Georgia. and additional mailing offices. HVAC nor may any pari of this book be reproduced. stored in a reieval system, or transmitted in any form or by any means-electronic, photocopying. recording, or other-without permission in writing from ASHRAE. Abstractr-Abstracted
9、and indexed by Engineering Information, Inc. Available electronically on Compendex Plus and in print in Engineer- ing Index. Disclaimer-ASHRAE has compiled this publication with care, but ASHRAE has not investigated. and ASHRAE expressly disclaims any duty to investigate, any producl service, proces
10、s, procedure. design, or the like which may be described herein. The appearance of any techni- cal data or editorial material in this publication does not constitute endorsement, warranty, or guaranty by ASHRAE of any producl ser- vice, process, procedure, design. or the like. ASHRAE does not warran
11、t that the information in this publication is free of errors, and ASHRAE does not necessarily agree with any statement or opinion in this publica- tion. The entice risk of the use of any information in this publication is assumed by the user. Postmasier-Send form 3579 10. HVAC when a research progra
12、m nears completion, writing a manuscript can begin, and the system is put in motion. Here the output is a manuscript itself. There are many inputs, comprising all the materi- als necessary for the paper. The system consists of various items of hardware such as a word processor, printer, stationary a
13、nd software, including the authors ideas, past experiences, influ- ences, and the researchers individual brains, all having their own characteristics. There are many opportunities for “malfunctions” during the production process: computers can break 181 - - - STD-ASHRAE SRCH IJHVAC 3-3-ENGL 1997 075
14、9b50 052803b T47 W 182 HVAC but not necessarily in that order. It should be remembered that, nowadays, abstracts may be used by various abstracting services that are widely disseminated, so that, in many cases, authors abilities are evaluated by their abstracts to a greater extent than ever before.
15、Some authors may complain about slow reviews, but sometimes the authors are partly respon- sible for this when the reviewers are discouraged by the large amount of time it can take to make corrections to a cumbersome, or flawed manuscript. In general, the higher the quality of a manu- script, and it
16、s general presentation, the quicker the review process is accomplished. It is better for the authors to spend more time finding “malfunctions” in their manuscripts, and making “improvements” before submission, than for the reviewers to spend proportionately more time at the evaluation stage. In conc
17、luding this editorial, I should like to make the following recommendations to authors who would like to see their manuscripts sail through the review process, and be accepted as sub- mitted, or with only minor revisions: 1. The introduction is rather more important than is generally supposed. 2. Sho
18、wing the originality of the study and how it advances the state of the art is fundamental to 3. Attractive results with a complete analysis always generates respect. the method section. STD-ASHRAE SRCH IJHVAC 3-3-ENGL 1997 W 0759b50 0528037 985 VOLUME 3, NUMBER 3, JULY 1997 i 83 4. Remember that man
19、y readers only read abstracts and conclusions. 5. Excellent papers come with many references. 6. Like the flowers in the field, add a bit of fragrance to your manuscript. The International Journal of HVAC these criteria take the form of simple ratios that include the rate constants (j and f) and geo
20、metric parameters for the heat exchanger. Unfortunately, all energy-based PEC share a common limitation: energy-based criteria place equal weight on mechanical work and heat transfer interactions. Fur- thermore, simple energy-based PEC do not provide guidance as to the tradeoffs between first Nicole
21、 DeJong and Mark Gentry are graduate research assistants and Anthony Jacobi is an assistant professor in the department of Mechanical unfortunately, this approach is very complex and expensive to implement. Furthermore, a full thermoeconomic analysis may require information that is difficult to obta
22、in early in the design process. Witte (1988) presents a simplified thermo- economic evaluation of heat exchangers, but his method neglects fan power and the impact of component performance on system behavior. The purpose of the current paper is to present a simplified method for assessing the air-si
23、de performance of heat exchangers in air-conditioning and refrigeration systems. The technique relies on established entropy generation analysis techniques with rate-equation constraints and uses a performance measure that values heat duty and penalizes cycle exergy rejection and destruction. In con
24、trast to the earlier related work, the effects of component operation on the system exergy destruction are included in this method. The proposed method is easier to apply than a full thermoeconomic analysis, and it provides economic information early in the design process. METHOD In most air-conditi
25、oning and refrigeration systems, air flow through the heat exchangers is driven by fans. A typical fan-exchanger component is shown schematically in Figure 1. Con- sider a control volume that encloses the air in the heat exchanger and the fan. The heat exchanger surface is excluded from the control
26、volume; this heat transfer surface is considered to be at a uniform temperature T,. Such an assumption is consistent with earlier air-side evalua- tions that assume uniform surface temperature and perfect fins (Cowell 1990); otherwise, it may be imagined that T, is simply the representative average
27、surface temperature. In Bejans (1996) terminology, the heat exchanger surface represents the frontier of the control volume. Heat enters this control volume at a rate of q, through the frontier at T,. Furthermore, air flows into the control volume at a mass flow rate of m with constant specific heat
28、s and a temperature T1. Air exits the control volume at the same mass flow rate with a temperature T2. Shaft work is pro- vided to the fan at a rate of P. The direction of heat transfer in the schematic is compatible with the operation of a condenser in an air-conditioning or refrigeration system, b
29、ut this method can be applied with equal validity to an evaporator. Later, a condenser analysis is discussed as an example. Consistent with the application to a condenser, the thermodynamic dead state was taken as a temperature of TI and atmospheric pressure. - STD*ASHRAE SRCH IJHVAC 3-3-ENGL 1997 U
30、 0757b50 05280iO 477 m VOLUME 3, NUMBER 3, JULY 1997 187 Control volume 6. T Figure 1. Fan and heat exchanger arrangement for air-conditioning system (The control volume is selected such that it does not include the surface of the heat exchanger; therefore, he heat interaction q enters the control v
31、olume at a boundary temperature of T,. Shaft power P is supplied to the control volume, and mass flows across control surfaces 1 and 2.) For the control volume shown in Figure 1, assuming steady state, negligible kinetic and potential energy changes, and uniform flow of a perfect gas at the inlet an
32、d exit control surfaces, the first law of thermodynamics provides the following relation: q+P = Cair(T2-T1) where Cair is the heat capacity rate of the flowing air stream and Cair = mc tions, the temperature rise across the fan, Ap/(pc,), is very small. transfer to be expressed in the following mann
33、er (Incropera and DeWitt 1996): For most applica- For refrigerant-to-air heat exchangers, adopting an E-N approach allows the rate of heat P where, from the definition of the Colburn j-factor, and assuming a constant refrigerant temperature, Equation (2) is completely valid only in the condensing zo
34、ne of the condenser and not in the desuperheating and subcooling zones. However, Stoecker and Jones (1982) report that this equa- tion provides reasonably accurate results throughout the entire condenser. The definitions of the Reynolds number, the hydraulic diameter, and the volumetric flow rate ca
35、n be rearranged to take the following forms (Kays and London 1984): and STD.ASHRAE SRCH IJHVAC 3-3-ENGL 1797 0757b50 05280Y1 30b 188 HVAC the unavoidable generation of entropy results in exergy destruction in the amount of Wlost = As - hd. On this basis, others have formulated second-law efficiencie
36、s such as when the irreversibilities equal the supplied exergy, no net exergy appears in the air stream and the efficiency vanishes. The use of any flow exergy sup- plied to the air during this process is destroyed in the environment through external irreversibili- ties such as direct-contact heat t
37、ransfer. Recognizing that this practice prevails, a condenser performance measure should not value Ad. The second flaw in the lost ability to convert energy to work is the operating cost of the heat exchanger. This operating cost can be assessed in eco- nomic terms if the marginal cost of fuel is kn
38、own (London 1982). In assessing component per- formance, value should not be ascribed to exergy that is destroyed externally during normal machine operation. Furthermore, the coupling between system and component behavior should be considered when evaluating component performance; component operatio
39、n affects machine-internal irreversibilities. With the conservation and rate equations, Equations (1) through (9), and the second-law measures of Equations (1 l), (12), and (13), a complete model is in place for assessing heat exchanger performance. The model has seven degrees of freedom and require
40、s specific component and system performance knowledge Equations (8), (9), and (13), respectively. Nevertheless, this performance evaluation method is relatively easy to implement and provides meaningful evaluative information. An example of implementation is provided in the next section. IMPLEMENTAT
41、ION however, the use of II would still be possible following the equations provided previously. The following example was used to illustrate the procedures. An offset strip-fin condenser with CJ = 0.816, T, = 25 “C, q = 3000 W, and AT= 70 m2 was considered. The correlations of Joshi and Webb (1987),
42、 f = 8.12 Re and where IID, = 3.37 and a = O. 123, were used to establish performance curves of the form given in Equations (8) and (9). Finally, a curve fit to manufacturers catalog data for a 3 112 ton (12 kW) residential split system provided the following for use with Equation (13) in assessing
43、the sys- tem penalty associated with off-design condenser operation (a similar curve fit can be developed for any system data): where T, is in degrees Celsius. Calculation of the performance criterion II involves three degrees of freedom in a nonlinear set of equations; namely, Afr, L, and Q are fre
44、e variables. The nonlinear system of equations was solved using a Newton-Raphson technique, and the relative residuals were always less than Heat exchanger surface temperature and pumping power are shown in Figure 2 for a base- line heat exchanger (Afr = 1.4 m2 and L = 50 mm) operating over a range
45、of airflow rates. The figure shows that Pincreased to 1% of the specified heat duty at the highest airflow rates considered. With an increase in airflow, the surface temperature required to deliver the specified duty decreased asymptotically toward the ambient of 25C. Both the pumping power and sur-
46、 face temperature were nonlinear with airflow rate. Because the heat duty was fixed, the effect of STD-ASHRAE SRCH IJHVAC 3-3-ENGL 1777 0757b5CI CI528099 CIL5 VOLUME 3, NUMBER 3, JULY 1997 191 30 25 U g 20 a 15 bD c as T., approached TI, the heat contribution to A, decreased. However, because P incr
47、eased with airflow rate, these two effects competed. The result was that A, passed through a minimum with increasing airflow rate, as shown in Figure 3. Figure 3 also shows that AsYs decreased as the airflow rate increased. This behavior was expected from the results shown in Figure 2 and Equations
48、(1 3) and (1 6). While the trends for Asys reflected in Figure 3 were to be generally expected, the exact values depend on the base- line condition. Exergy destruction due to component effects on the system decreased with con- STDOASHRAE SRCH IJHVAC 3-3-ENGL 2997 0759b50 05280q5 T5L o !:/II! !I! HVA
49、Cr3 ; therefore, at high Q, where P dominated, an exchanger with a large frontal area performed better than one with a small frontal area, For a given Q, as Afr increased, the air velocity and Reynolds number decreased, resulting in an increase int However, for high Q the change infwith Reynolds number was small, and hence the effect of the change in Afr on pumping power was greater than the effect of the change infi For the family of curves shown in Figure 4A, the preferred flow rate decreased as A
