1、Load Calculation Applications Manual Second Edition9 781936 504756ISBN 978-1-936504-75-6Product code: 90662 12/14The Applications-Oriented Resource for Load CalculationsThis new edition of Load Calculation Applications Manual presents two methods for calculating design cooling loadsthe heat balance
2、method (HBM) and the radiant time series method (RTSM)in a thorough, applications-oriented approach that includes extensive step-by-step examples for the RTSM. Updates for this edition reflect changes in lighting, materials, and equipment used in buildings today, as well as new methods available sin
3、ce the first edition, including New internal heat gain data for office equipment New methods and data for the effects of internal shading on solar heat gains New data on heat gains from kitchen equipment, based on experimental measurements New weather data for more than 6000 stations worldwide A new
4、 ASHRAE clear-sky model, applicable worldwide Improved methods for generating design day temperature profiles A major revision of thermal properties data for building materialsThis essential engineering reference begins with an overview of heat transfer processes in buildings and a discussion of how
5、 they are analyzed together to determine the HVAC load. Later chapters give in-depth coverage of the RTSM and HBM theory and application, systems and psychrometrics, and heating loads.With this book comes access to spreadsheets for computing cooling loads with the RTSM and calculating the solar irra
6、diation, conduction time factor series, and radiant time factors used in the method. The spreadsheets can be adapted to compute cooling loads for a wide range of buildings.Jeffrey D. SpitlerLoad Calculation Applications Manual (I-P)Spitler1791 Tullie Circle NE | Atlanta, GA 30329-2305www.ashrae.orgR
7、P-1616I-P EditionA complete reference including Heat transfer processes and analysis New data and methods Applications-oriented, step-by-step examples Heat balance and radiant time series methodsSecond EditionLoad Calculation Applications Manual Cover IP 2nd Ed.indd 1 11/6/2014 2:07:03 PMLoad Calcul
8、ationApplications ManualSecond EditionI-P EditionThis publication was supported by ASHRAE Research Project RP-1616 andwas prepared under the auspices of TC 4.1, Load Calculation Data and Procedures.Results of cooperative research between ASHRAE and Oklahoma State University.About the AuthorJeffrey D
9、. Spitler, PhD, PE, is Regents Professor and OG the weather files provided the needed weather information to per-form load calculations around the world. If the files or information at the link are notaccessible, please contact the publisher.1.1 Definition of a Cooling LoadWhen an HVAC system is ope
10、rating, the rate at which it removes heat from a space isthe instantaneous heat extraction rate for that space. The concept of a design cooling loadderives from the need to determine an HVAC system size that, under extreme conditions,will provide some specified condition within a space. The space se
11、rved by an HVAC sys-tem commonly is referred to as a thermal zone or just a zone. Usually, the indoor boundarycondition associated with a cooling load calculation is a constant interior dry-bulb temper-ature, but it could be a more complex function, such as a thermal comfort condition. Whatconstitut
12、es extreme conditions can be interpreted in many ways. Generally, for an office itwould be assumed to be a clear sunlit day with high outdoor wet-bulb and dry-bulb tem-peratures, high office occupancy, and a correspondingly high use of equipment and lights.Design conditions assumed for a cooling loa
13、d determination are subjective. However, afterthe design conditions are agreed upon, the design cooling load represents the maximumor peak heat extractionrate under those conditions.1.2 The Basic Design QuestionsIn considering the problem of design from the HVAC system engineers viewpoint, adesigner
14、 needs to address the following three main questions:1. What is the required equipment size?2. How do the heating/cooling requirements vary spatially within the building?3. What are the relative sizes of the various contributors to the heating/cooling load?The cooling load calculation is performed p
15、rimarily to answer the second question,that is, to provide a basis for specifying the required airflow to individual spaces within theT2Load Calculation Applications Manual (SI), Second Editionbuilding. The calculation also is critical to professionally answering the first question.Answers to the th
16、ird question help the designer make choices to improve the performanceor efficiency of the design and occasionally may influence architectural designers regard-ing energy-sensitive consequences.1.3 Overview of the ASHRAE Load Calculation Methods1.3.1 Models and RealityAll calculation procedures invo
17、lve some kind of model, and all models are approxi-mate. The amount of detail involved in a model depends on the purpose of that model.This is the reality of modeling, which should describe only the variables and parametersthat are significant to the problem at hand. The challenge is to ensure that
18、no significantaspects of the process or device being modeled are excluded and, at the same time, thatunnecessary detail is avoided.A complete, detailed model of all of the heat transfer processes occurring in abuilding would be very complex and would be impractical as a computationalmodel, even toda
19、y. However, building physics researchers and practitioners gener-ally agree that certain modeling simplifications are reasonable and appropriateunder a broad range of situations. The most fundamental of these is that the air inthe space can be modeled as well-stirred. This means there is an approxim
20、ately uni-form temperature throughout the space due to mixing. This modeling assumption isquite valid over a wide range of conditions. With that as a basis, it is possible to for-mulate fundamental models for the various heat transfer and thermodynamic pro-cesses that occur. The resulting formulatio
21、n is called the HBM. There is anintroduction to the general principles of the HBM in Chapter 2 and further descrip-tion in Chapter 11.1.3.2 The Heat Balance MethodThe processes that make up the heat balance model can be visualized using the sche-matic shown in Figure 1.1. It consists of four distinc
22、t processes:1. Outside face heat balance2. Wall conduction process3. Inside face heat balance4. Air heat balanceFigure 1.1 shows the heat balance process in detail for a single opaque surface. Theshaded part of the figure is replicated for each of the surfaces enclosing the zone.The process for tran
23、sparent surfaces is similar to that shown but does not have theabsorbed solar component at the outside surface. Instead, it is split into two parts: aninward-flowing fraction and an outward-flowing fraction. These fractional parts partici-pate in the inside and outside face heat balances. The transp
24、arent surfaces, of course, pro-vide the transmitted solar component that contributes to the inside heat balance.The double-ended arrows indicate schematically where there is a heat exchange, andthe single-ended arrows indicate where the interaction is one way. The formulation of theheat balance cons
25、ists of mathematically describing the four major processes, shown asrounded blocks in the figure.1.3.3 The Radiant Time Series MethodThe RTSM is a relatively new method for performing design cooling load calcula-tions. It is derived directly from the HBM and effectively replaced all other simplified
26、(non-heat-balance) methods such as the transfer function method (TFM), the coolingIntroduction3load temperature difference/solar cooling load/cooling load factor method (CLTD/SCL/CLFM), and the total equivalent temperature difference/time averaging method (TETD/TAM). The RTSM was developed in respon
27、se to a desire to offer a method that was rig-orous yet did not require the iterative calculations of the previous methods. In addition,the periodic response factors and radiant time factors have clear physical meanings;when plotted, they allow the user to visually see the effects of damping and tim
28、e delayon conduction heat gains and zone response.The utility of the RTSM lies in the clarity, not the simplicity, of the procedure.Although the RTSM uses a “reduced” heat balance procedure to generate the radianttime series (RTS) coefficients, it is approximately as computationally intensive as the
29、heat balance procedure upon which it is based. What the RTS method does offer isinsight into the building physics without the computational rigor of the HBM, a sacri-fice in accuracy that is surprisingly small in most cases. Previous simplified methodsrelied on room transfer function coefficients th
30、at completely obscured the actual heattransfer processes they modeled. The heat-balance-based RTS coefficients, on theother hand, provide some insight into the relationship between zone construction andthe time dependence of the building heat transfer processes. The RTSM abstracts thebuilding therma
31、l response from the fundamentally rigorous heat balance and presentsthe effects of complex, interdependent physical processes in terms that are relativelyeasy to understand. The abstraction requires a number of simplifying assumptions andapproximations. These are covered in Section 7.1. Figure 1.2 s
32、hows the computationalprocedure that defines the RTSM. A more detailed schematic is shown in Chapter 7.In the RTSM, a conductive heat gain for each surface is first calculated using air-to-air response factors. The conductive heat gains and the internal heat gains are then splitinto radiant and conv
33、ective portions.All convective portions are instantaneously convertedto cooling loads and summed to obtain the fraction of the total hourly cooling load causedby convection.Figure 1.1 Schematic of heat balance process in a zone.(Source: Cooling and Heating Load Calculation Principles, Figure 1.1).4L
34、oad Calculation Applications Manual (SI), Second EditionRadiant heat gains from conduction, internal sources, and solar transmission are oper-ated on by the RTS to determine the fraction of the heat gain that will be converted to acooling load in current and subsequent hours. These fractional coolin
35、g loads are added tothe previously calculated convective portions at the appropriate hour to obtain the totalhourly cooling load.1.4 Organization of the ManualThis manual is organized to roughly proceed from the general to the specific.Chapter 2 provides an overview of the heat transfer processes pr
36、esent in buildings and abrief discussion of how they are analyzed together in order to determine the building cool-ing load. Chapters 36 cover thermal properties, design conditions, infiltration, and inter-nal heat gainsall of which are relevant to all load calculation methods. Chapters 7 and 8cover
37、 the theory and application of the RTSM. Chapter 9 covers systems and psychromet-rics, analyses of which are necessary to determine equipment sizes. Chapter 10 considersheating load calculations. Chapter 11 covers the HBM and its implementation.Throughout the manual, numerous shaded examples are pre
38、sented to illustrate vari-ous aspects of the RTSM. A number of the examples are performed using spreadsheetsthat are included in the supporting files online at www.ashrae.org/lcam.ReferencesPedersen, C.O., D.E. Fisher, J.D. Spitler, and R.J. Liesen. 1998. Cooling and HeatingLoad Calculation Principl
39、es. Atlanta: ASHRAE.Figure 1.2 Schematic of the radiant time series method.(Source: Cooling and Heating Load Calculation Principles, Figure 1.2).2Fundamentals of Heat Transfer and Thermodynamics5he cooling load is defined as the amount of heat that must be removedfrom the room air to maintain a cons
40、tant room air temperature. Con-versely, the heating load is the amount of heat that must be added to theroom air. To determine these quantities, it is necessary to estimate theheat transmission into or out of the room. In turn, this requires analysis ofall three modes of heat transferconduction, con
41、vection, and radiation within thebuilding envelope and between the building envelope and its surroundings. (Here, theterm building envelope refers to the walls, roofs, floors, and fenestrations that makeup the building.)The three modes of heat transfer all occur simultaneously, and it is the simulta
42、ne-ous solution of all three modes of heat transfer that complicates the analysis. In practice,this simultaneous solution is done with a computer program either during the load cal-culation procedure (e.g., the heat balance method HBM) or prior to the load calcula-tion procedure (all simplified load
43、 calculation procedures rely on tabulated factors thatwere developed with a simultaneous solution of all three modes of heat transfer).Before concerning ourselves with the simultaneous solution, we should first con-sider the three modes independently. For convection and radiation, the treatment ofth
44、e individual modes of heat transfer does not go far beyond what is taught in a firstundergraduate course1in heat transfer. For steady-state conduction heat transfer, asused in heating load calculations, this is also the case. For transient conduction heattransfer, as used in cooling load calculation
45、s, the derivation of the solution procedurecan be somewhat complex, although its application, in practice, is not very difficult.Each of the three modes is discussed briefly below. Then, after considering thethree modes of heat transfer, the simultaneous solutionbased on the first law ofthermodynami
46、csis briefly discussed.2.1 ConductionSteady StateHeat transfer through building walls and roofs is generally treated as a pureconduction heat transfer process, even though, for example, convection and radia-tion may be important in an internal air gap2in the wall. Conduction is the trans-fer of heat
47、 through a solid3via random atomic and molecular motion in responseto a temperature gradient. Elements of the building envelope such as thermalbridges and corners distort the temperature gradients so that the heat flows indirections other than purely perpendicular to the envelope surfaces. Such heat
48、flow is said to be multidimensional. For building load calculations, multidimen-sional conduction heat transfer is generally approximated as being one-dimen-sional; however, the approximations do take into account the impact of thermalbridges.4Heat loss from foundation elements is also multidimensio
49、nal, but again,approximations are made that simplify the calculation procedure.1. Cf. Incropera and DeWitt (2001).T2. Even though heat transfer in an air gap is due to convection and radiation, it is approximated as beinga conduction process with a fixed thermal resistance that is independent of the temperatures of thegap surfaces.3. Technically, conduction also occurs in liquids and gases, too. But here we are only concerned with con-duction in solids.4. Appendix E covers the treatment of thermal bridges.6Load Calculation Applications Manual (SI), Second EditionOn
