Space Loads PDF - Chapter 5

Chapter5.fm Page 157 Monday, November 29, 2021 11:56 AM Chapter 5 SPACE LOADS In this chapter, the fundamental elements that accompany th...

Chapter5.fm Page 157 Monday, November 29, 2021 11:56 AM Chapter 5 SPACE LOADS In this chapter, the fundamental elements that accompany the space load calculations for determining the required supply air temperatures and airflow rates are presented. The material includes the evaluation of the overall coefficients of heat transfer for building components, the methodology for calculation of space heat gain and loss, and the application of the resulting information to system design decisions. Chapters 4, 15, 17, 18, 26 and 27 of the 2021 ASHRAE Handbook—Fundamentals are the major sources of information. 5.1 Introduction Heating and cooling loads are the rates of energy input (heating) or removal (cooling) required to maintain an indoor environment at a desired combination of temperature and humid- ity. The basic components of heating and cooling loads are illustrated in Figure 5-1. Heating and cooling systems are designed, sized, and controlled to accomplish this energy transfer. The amount of heating or cooling required at any particular time varies widely, depending on exter- nal (e.g., outside temperature) and internal (e.g., number of people present) factors. Peak design heating and cooling load calculations determine the maximum rate of heating and cooling energy transfer needed at any time in the year. Heating and cooling load calculations are the primary basis for the design and selection of most heating and air-conditioning systems and components. These calculations are necessary to determine the size of piping, ducting, diffusers, air handlers, boilers, chillers, coils, compres- sors, fans, and every other component of the systems that condition indoor environments. Cool- ing and heating load calculations will directly or indirectly affect the first cost of building construction, the comfort and productivity of building occupants, and the system operation and energy consumption. 5.2 Background Heating requires a calculation of the heat loss, whereas cooling addresses the amount of heat gain. The general concepts are similar in that heat transfer is the basis of the calculations. Heat Fig. 5-1 Components of Heating and Cooling Loads Chapter5.fm Page 158 Monday, November 29, 2021 11:56 AM 158 Principles of HVAC, 9th Edition transfer is determined by establishing the area of surface of separation, the quality of the mate- rials used, and the difference in temperature on either side of the surface. Although heating and cooling load calculations are similar, there are inherent differences. Cooling needs to account for all the heat gains experienced in the space to determine the max- imum cooling required by the system. This includes the heat that is transmitted through the building envelope (walls, windows, roof, floor, etc.) or any surface that has a temperature dif- ferential, as well as the heat generated within the space. Beyond the envelope, internal heat gain contributed by people, equipment, and lights have to be quantified. A heat loss calculation on the other hand is conducted to size a heating system. This system must be sized to compensate for the heat loss to maintain the space temperature. The heat loss calculation is simpler than heat gain because its worst-case typically occurs at night when the outdoor air temperatures are low- est, the number of items generating heat within the building are lower (no or limited occupancy, fewer lights on and equipment not operating), and there is no solar heat generated by the sun. Because some of these influences are eliminated, the calculation is simplified. This chapter out- lines the procedure for both heating and cooling load calculations, while distinguishing the dif- ferences as well as outlining in detail the methods used to quantify each of the heat loss/gain components. To assist in user comprehension, the chapter is organized to address heating, the simpler of the calculations first. Both system and space level calculations are required for heating and cooling calculations. Space level refers the load experienced by the individual occupied space or room and the heating and cooling required to maintain its specified conditions (temperature and humidity). This chap- ter focuses on the space level of calculation. The results of space level calculations are important because they provide essential information for the system level calculation. System level refers to the need to size an entire system which has the potential to consist of a single space or multiple spaces. The system level analysis and the differences between it and space level loads is dis- cussed in Chapter 6. Due to the complexity of buildings, there are many variables that influence the load calcu- lation impacting the process and the results. Since these loads are used to size equipment, a designer must understand the effect of their decisions as they proceed in the calculation process. A simple example would be the decision regarding which outdoor temperatures to choose for the calculations. This value is going to depend on the needs of the owner for a specific application. If the hottest summer outdoor temperatures (dry bulb and wet bulb) on record are selected, it is likely in conjunction with other conservative assumptions (building fully occupied, all the lights on, all equipment is operating, etc.) that equipment selected based on this load will always meet the demand for cooling. But, it is understood that these temperatures will likely occur very few hours during the year, if at all. This means the equipment selected is larger than it needs to be a good portion of the time. This oversizing will allow for adequate sensible cooling of the space but will come at a potential expense—humidity control, equipment that is physically larger than necessary, equipment that is more expensive in first costs, equipment that may not operate as efficiently as desired, equipment that may cycle on and off more to maintain space conditions shortening the equipment life, etc. For a critical environment facility in which space conditions cannot fluctuate (such as laboratories, operating rooms, certain manufacturing processes) this added expense for large equipment is acceptable, but for a space that can allow some fluctuation in indoor temperature and humidity, using lower outdoor temperatures may be more appropriate to keep the system economics more reasonable. For this reason, designers must have discussions with building owners to better understand their needs and expectations. Even after consultation with an owner, there will be a need for engineering judgment. Conducting a proper cooling load calculation gives values adequate for equipment selection. Understanding that this is an estimation rather than an exercise in precision provides context. Variation in the heat transmission coefficient of typical building materials and composite assem- blies, the differing motivations and skills of those who physically construct the building, and the manner in which the building is actually operated are some of the variables that make a numer- ically precise calculation impossible. While the designer uses reasonable procedures to account for these factors, the calculation can never be more than a good estimate of the actual cooling Chapter5.fm Page 159 Monday, November 29, 2021 11:56 AM Chapter 5 Space Loads 159 load. This will ultimately afford the designer the opportunity to evaluate many different options when it comes to equipment selection taking cost and availably into account in the process. It is important to clarify that the content of this text focuses on commercial building appli- cations, which differs slightly from residential for load calculations. Residences and small com- mercial buildings have heat gains and cooling loads that are dominated by the building envelope (walls, roof, windows, and doors), whereas internal gains from occupants, lights, equipment, and appliances play a significant role and often dominate in commercial buildings. Direct appli- cation of nonresidential methods results in unrealistically high cooling loads for residential applications. With respect to heating and cooling load calculation and equipment sizing, the unique fea- tures distinguishing residences from other types of buildings are the following: Smaller Internal Heat Gains. Residential system loads are primarily influenced by heat gain or loss through structural components and by air leakage or ventilation. Internal heat gains, particularly those from occupants and lights, are small compared to those in commercial or industrial structures. Varied Use of Spaces. Use of spaces in residences is more flexible than in commercial buildings. Localized or temporary temperature variances are often tolerable. Fewer Zones. Residences are generally conditioned as a single zone or, at most, a few zones. Typically, a thermostat located in one room controls unit output for multiple rooms, and capacity cannot be redistributed from one area to another as loads change over the day. This results in some hour-to-hour temperature variation or “swings.” Greater Distribution Losses. Residential ducts are frequently installed in attics or other unconditioned buffer spaces. Duct leakage and heat gain or loss can require significant increases in unit capacity. Residential distribution gains and losses cannot be neglected or estimated with simple rules of thumb. Dehumidification Issues. Dehumidification occurs during cooling unit operation only, and the space condition control is usually limited to use of room thermostats (sensible heat-actuated devices). Excessive sensible capacity results in short-cycling and severely degraded dehumidification performance. Variations in the characteristics of residences can lead to surprisingly complex load calcula- tions. Time-varying heat flows combine to produce a time-varying load. The relative magnitude and pattern of the heat flows depends on the building characteristics and exposure, resulting in a building-specific load profile. In general, an hour-by-hour analysis is required to determine that profile and find its peak. In theory, cooling and heating processes are identical; a common analysis procedure should apply to either. Acceptable simplifications are possible for heating; however, for cooling, dif- ferent approaches are used. If calculating residential cooling loads, details of this procedure can be found in Chapter 17 of the 2021 ASHRAE Handbook—Fundamentals. This chapter should not be considered all-inclusive but rather an introduction to the proper process for determining space heating and cooling loads in commercial buildings. It should be used in conjunction with other space heating and air-conditioning fundamental found in this text. More detailed technical information on this topic to enhance the content of this text can be found in the 2021 ASHRAE Handbook—Fundamentals. 5.3 Heat Transfer Coefficients 5.3.1 Modes of Heat Transfer The design of a heating, refrigerating, or air-conditioning system, including selection of building insulation or sizing of piping and ducts, or the evaluation of the thermal performance of system parts such as chillers, heat exchangers, and fans, is based on the principles of heat transfer given in Chapter 4, Heat Transfer, of the 2021 ASHRAE Handbook—Fundamentals.