1、AIR-CONDITIONING SYSTEM DESIGN MANUALSECOND EDITIONAIR-CONDITIONING SYSTEM DESIGN MANUALSECOND EDITIONWalter T. Grondzik, EditorAIR-CONDITIONING SYSTEM DESIGN MANUALThe Air-Conditioning System Design Manual was written to assist entry-level engineers in the design of air-conditioning systems. It is
2、also usablein conjunction with fundamental HVAC thus, such information is notrepeated in Chapter 7. Chapter 9 covers a variety of specialHVAC other-wise there is no connection between design intent and design deci-sions. Pre- and post-occupancy validations are also important toensure that the constr
3、uction process and ensuing operational proce-dures have delivered design intent. Such validations are a keyaspect of building commissioning.2.2 DESIGN VERSUS ANALYSIS Anyone who has taken a course in mathematics or any of thephysical sciences is familiar with the process of analysis. In a typi-cal a
4、nalysis, a set of parameters is given that completely describes aproblem, and the solution (even if difficult to obtain) is unique.There is only one correct solution to the problem; all other answersare wrong.Design problems are inherently differentmuch different. Adesign problem may or may not be c
5、ompletely defined (some of theparameters may be missing) and there are any number of potentiallyacceptable answers. Some solutions may be better than others, butthere is no such thing as a single right answer to a design problem.There are degrees of quality to design problem solutions. Somesolutions
6、 may be better (often in a qualitative or conceptual sense)than others from a particular viewpoint. For a different context orclient, other solutions may be better. It is important to clearlyunderstand the difference between analysis and design. If you areused to looking for the correct answer to a
7、problem (via analysis),and are suddenly faced with problems that have several acceptableanswers (via design), how do you decide which solution to select?Learn to use your judgment (or the advice of experienced col-leagues) to weigh the merits of a number of solutions that seem towork for a particula
8、r design problem in order to select the bestamong them.Figures 2-1 and 2-2 illustrate the analysis and design pro-cesses, respectively. Analysis proceeds in a generally unidirectionalflow from given data to final answer with the aid of certain analyti-cal tools. Design, however, is an iterative proc
9、ess. Although thereare certain “givens” to start with, they are often not immutable butAIR-CONDITIONING SYSTEM DESIGN MANUAL11subject to modification during the design process. For example, anowner or architect may be confronted with the energy implicationsof excessively large expanses of glass that
10、 had been originally spec-ified and may decide to reduce the area of glazing or change theglazing properties. The mechanical designer may try various sys-tem components and control strategies before finding one that bestsuits the particular context and conditions. Thus, design consists ofa continuou
11、s back-and-forth process as the designer selects from auniverse of available systems, components, and control options tosynthesize an optimum solution within the given constraints. Thisiterative design procedure incorporates analysis. Analysis is animportant part of any design.Since the first step i
12、n design is to map out the general boundar-ies within which solutions are to be found, it may be hard to knowwhere and how to start because there is no background from whichto make initial assumptions. To overcome this obstacle, makeinformed initial assumptions and improve on them through subse-quen
13、t analysis. To assist you in making such initial assumptions,simple rules are given throughout the chapters in this manual, andillustrative examples are provided in the appendices.2.3 DESIGN PHASESA new engineer must understand how buildings are designed.Construction documents (working drawings and
14、specifications)Figure 2-1. Diagram illustrating analysis.12 THE DESIGN PROCESSfor a building are developed as a team effort. The architect usuallyacts as the prime design professional and project coordinator,although experienced owners and developers may deal directlywith pre-selected HVACheating an
15、d cooling degree-days/hours;design wind velocity (and direction) for winter and summer;applicable zoning, building, mechanical, fire, and energycodes; andrate structure, capacity, and characteristics of available utilitiesand fuels.Additional information regarding solar radiation availabilityand sub
16、surface conditions would be included if use of a solar ther-mal system or ground-source heat pump was anticipated.The environmental conditions to be maintained for each build-ing space should be defined bydry-bulb and wet-bulb temperatures during daytime occupiedhours, nighttime occupied hours, and
17、unoccupied hours;ventilation and indoor air quality requirements;any special conditions, such as heavy internal equipment loads,unusual lighting requirements, noise- and-vibration-free areas,humidity limits, and redundancy for life safety and security; andacceptable range of conditions for each of t
18、he above.An understanding of the functional use for each area is essen-tial to select appropriate HVAC fuel and/or electrical use and thermal storagerequirements; initial and life-cycle costs; acoustical requirementsand capabilities; compatibility with the building plan and the struc-tural system; a
19、nd the effects on indoor air quality, illumination, andaesthetics. Also consider energy code compliance and green designimplications (as appropriate).Early in the design phase, the HVAC&R designer may be askedto provide an evaluation of the impact of building envelope designoptions (vis-vis energy c
20、ode compliance and trade-offs and/orgreen building intents), heavy lighting loads (i.e., more than 2 W/ft222 W/m2), and other unusual internal loads (i.e., more than 4 W/ft243 W/m2) on HVAC system performance and requirements. Ques-tions should also be expected regarding the optimum location ofmajor
21、 mechanical equipmentconsidering spatial efficiency, sys-tem effectiveness, aesthetics, and acoustical criteria. Dependingupon the level of information available, the designer may be asked toprepare preliminary HVAC system sizing or performance estimatesbased upon patterns developed through experien
22、ce or based uponresults from similar, previously designed projects. Some design esti-16 THE DESIGN PROCESSmates that may be useful for a first cut are given in Table 2-1. Addi-tional values appropriate for this design phase can be found in theASHRAE Pocket Guide for Air Conditioning, Heating, Ventil
23、ation,Refrigeration (ASHRAE 2005c).If envelope and internal loads are reasonably well defined,peak load and rough energy calculations for alternative HVAC sys-tems may be prepared at this time using appropriate methods forpresentation to the architect and/or owner. Although they are pre-liminary and
24、 will change as the building design proceeds, such pre-liminary loads are usually definitive enough to compare theperformance of alternative systems because these systems will besized to meet the same loads. As you gain experience, you will beable to estimate the likely magnitude of the loads for ea
25、ch area in abuilding with a little calculation effort.Resources useful during this phase of design include designmanuals, textbooks, equipment literature, and data from existinginstallations. Frequently, this type of early system evaluationeliminates all but a few systems that are capable of providi
26、ng theenvironmental requirements and are compatible with the build-ing structure.If the client requests it, if architectural details have been suffi-ciently developed, and if the mechanical engineers fee has been setat a level to warrant it, comparisons between construction (first)costs and operatin
27、g (life-cycle) costs and the performance of differ-ent HVAC&R systems can be made in greater detail. Typically, onesystem is set as a reference (or base) and other proposed systemsare compared to this base system. Such an analysis would proceedaccording to the following steps:1. Estimate the probabl
28、e capital costs of each system using unitarea allowances, a rough selection of equipment, sketches ofsystem layouts, and such tools as:Cost-estimating manualsRecently completed similar projects (many technical jour-nals contain case studies that provide such information)Local HVAC&R contractorsProfe
29、ssional cost estimatorsDesign office filesExperienced design engineers.2. Identify the energy source or sources available and their costper a convenient unit of energy (million Btu, kWh, therm), con-sidering both present and anticipated costs. Determine localAIR-CONDITIONING SYSTEM DESIGN MANUAL17Ta
30、ble 2-1. Selected Load andAirflow Estimates for Schematic DesignGeneral:450 100 ft2/ton for cooling loads123m2/kW1.5 cfm/ft2air supplyexterior spaces7.6 L/s per m20.75 cfm/ft2air supplyinterior spaces (minimum)3.8 L/s per m2400 cfm/ton air supplyfor all-air systems54 L/s per kWOffices: 500 ft2/ton 1
31、3 m2/kWbased upon:lights1.5 W/ft216 W/m2fans0.75 W/ft28 W/m2pumps0.25 W/ft22.7 W/m2miscellaneous electrical2.0 W/ft221 W/m2occupancy150 ft2/person14 m2/personHigh-RiseApartment Buildings:1000 ft2/ton for north-facingapartments 26 m2/kW500 ft2/ton 13 m2/kW for othersHospitals: 333 ft2/ton based upon1
32、000 ft2/bed8.7 m2/kW at 93 m2/bedShopping Centers: average400 ft2/ton10.4 m2/kWdepartment stores2 W/ft221 W/m2specialty stores5 W/ft254 W/m2Hotels: 350 ft2/ton 9.1 m2/kWRestaurants: 150 ft2/ton 3.9 m2/kWCentral Plants:Urban districts 380 ft2/ton 9.9 m2/kWCollege campuses 320 ft2/ton 8.3 m2/kWCommercial centers 475 ft2/ton 12.4 m2/kWResidential centers 500 ft2/ton 13 m2/kW
