Today’s discussion will include comments on material from Chapters 6 & 7 of the Principles of HVAC. Chapter 6 serves to synthesize the material from previous chapters. In particular, the chapter provides a basis to analytically determine sensible heat loads from internal and external sources. The subject of calculating internal and external loads includes considerations of both convective and radiant loads on a structure. To simplify this process, we first consider wintertime heating loads. Winter Heating Loads Primarily Heating Loads Coldest days are often overcast so that radiant loads are minimal and can often be neglected. Heating Factors 1. Factors which can be treated by evaluating overall heat transfer coefficients, U, so that
Q U AT
Walls Roofs & Ceilings Windows Basement walls Floors (above & below grade)
2. Factors that depend on wall perimeter Slab Floors Q F2 P T 3. Infiltration/Exfiltration Losses
QS mc p T
QL mh fg W
Before we discuss these in some detail, perhaps we should lay down certain general standards as to how material properties should be found. In selecting the design indoor conditions we have included no conservatism in the selection of Tr and Wr. To ensure that the system works well, we should introduce certain conservatism into our selection of material properties. Wood studs or siding. If the material is not specified, choose a suitable material that has the highest thermal conductivity so as to maximize heat loads. For example, when selecting a material for studs or rafters, Table 4, Ch 24 of the Fundamentals lists several woods which can be used: fir, pine, spruce, hemlock or larch.
Wood Southern Pine Douglas Fir-Larch Hem-Fir, Spruce-Pine-Fir
k, BTU/hr ft oF 1.00-1.12 0.95-1.01 0.74-0.90
Of these, southern pine has the largest thermal conductivity. Without knowing which wood is to be used in a design, introduce conservatism by designing with the largest value, i.e. 1.12. Concrete, Concrete Block or Brick: These materials come in various densities. Brick, for example, comes in densities ranging between 70 and 150 lb/ft3. The higher density material, with fewer air spaces, has the higher conductivity. Brick, fired clay 150 lb/ft3 110 lb/ft3 70 lb/ft3 k, BTU/hr ft oF 8.4-10.2 4.9-5.9 2.5-3.1
Again, unless this material is specified, choose the highest conductivity in order to be conservative, i.e. 10.2 BTU/hr ft oF. If 110 lb/ft3 were specified, choose a value of 5.9 BTU/hr ft oF.
Infiltration losses: Infiltration losses are shown in Fig 10 & 11, Chapter 25, Fundamentals. Add some conservatism to the median values listed by going up one level. For New Construction, where the median infiltration rate is 0.5 air changes per hour, go up one level to 0.75 ACH. For low income housing, the median infiltration rate is 0.75 ACH; increase this to 1 ACH. (Note that these values are for an unoccupied dwelling.) On page 25.14 of the Fundamentals these values are to be increased by 0.10 to 0.15 ACH to account for occupancy. Occupancy, Lights, Equipment Loads: In accounting for all of the internal loads placed on a structure, one often encounters a number of loads that are of an intermittent nature. It is common practice to weight such loads with a factor between 0 and 1 to account for variability. A weighting factor of 0.5 is recommended. Large air spaces (in rafters) Table 3, Chapter 24 of the Fundamentals describes thermal resistances across air gaps in the range of 0.5 to 3.5 inches. For larger air gaps, use the same 3.5 inch value for heating, increase 3.5 in value linearly for cooling. Ceiling insulation: ceiling insulation is often specified in terms of R value. Unlike wall
insulation, sold in rolls, ceiling insulation is blown into the space in sufficient thickness to provide the specified value including the effects of any rafters or other materials. Citations used in problem solving. In courses corresponding to the entry level into the Department, I place considerable emphasis on having a professional appearance to homework. In particular, I require that all problems include a problem statement, a free body diagram, a list of principles applied, assumptions made, correlations applied and properties used. I ask that the correlations and properties include a citation as to where they were obtained. I’m sure that those of you from industry understand why I insist that solutions be given in a form that those unfamiliar with the material can understand the solution. (One consideration is that it allows a supervisor to understand what you’ve done.) I would like for you to cite correlations, coefficients from tables and other data using the Fundamentals book. You will need to look up a number of correlation coefficients in any case. By citing references from the Fundamentals, you will become familiar with the book and where information can be found. I think that it is important that you gain this familiarity. Also, I am giving consideration to allowing use of this reference during the final. Familiarity with the book would be of considerable
benefit in quickly finding information in a timelimited situation. Windows provide a significant contribution to heating and cooling loads. These are discussed in length in Chapter 20 of the Fundamentals Insulation is difficult. Generally limited to one or more “dead air” gaps between glazings. Frames are often constructed of aluminum or other high conductivity material. Special provisions are needed to limit significant heat transfer through these devices. One approach is to insert a low conductivity material between the inside and outside metal sections. This device is termed a thermal break. Storm windows are not explicitly described in Chapter 29. An adequate method of including such devices is to treat them as a double glazed window with a thermal break. Windows come in several styles including those that can be raised and those that cannot. Unless stated otherwise, assume that all windows are operable.
Design Project No. 2 EML 4601-Spring 2000 You are analyzing some design changes for a light commercial building to be located in Chicago, Illinois. Someone has selected materials for the walls, roof, windows, doors, floor, etc. A heating season energy estimate was also done. Floor size: 70x50 ft. Wall Area: 1920 ft2. Glass and Doors: 480 ft2 Annual Heating Estimate: 60000 BTU/ft2 year. Electric Heat Rate: 10¢/kWh Heat Loss Breakdown: Infiltration 19% Roof: 22% Glass & Doors: 20% Floor: 17% Walls: 22% Original Wall Design: Face Brick Vegetable fiberboard sheathing, 0.5in. 2x4 wood studs Gypsum plaster board 0.625 in (no insulation) Original Glass and Doors: U=1.1 BTU/hr ft2 F Investigate the economic feasibility of installing 3.5 inches of batt insulation and/or 3.5 inches of batt insulation
and 1 inch of glass fibre organic bonded board insulation in walls. The batt insulation costs 35 cents per square foot, and the board insulation costs 41 cents per square foot. Also consider the savings if going to an improved window/door such that U=0.59 BTU/hr ft2 F when costs are an additional $1 per square foot. If the original R for the ceiling was 19 and you could go to R=30 for an additional 5 cents per square foot, is it economically sound to do so? Prepare a written report with a maximum length of two pages with your recommendations and reasons. DUE: 2 April 2002. Homework, Chapter 6: 1, 4, 5, 6, 7, 10
Answers: 70.5·ΔT, 70.5·ΔT, 123·ΔT; 5700 BTU/hr, 9420 BTU/hr, 3320 BTU/hr; 219 cfm, 21200 BTU/hr, 4100 BTU/hr; 13500 BTU/hr, 783 BTU/hr, 340 BTU/hr, 2190 BTU/hr; 2362 W, 403W, 2765W; .51 BTU/hr·ft2·oR, 0.154 BTU/hr·ft2·oR, 1.27 BTU/hr·ft2·oR, 1.27 BTU/hr·ft2·oR, 62820 BTU/hr, 48820 BTU/hr
Chapter 7, PHVAC Problems in chapter 6 are limited to consideration of wintertime conditions. These analyses are simplified by the absence of a radiant load. Additionally, the methods introduced in this section are considerably simplified by treating all heat transfer as being steady state. While such treatment emphasizes the basic concepts, they fail to include two major factors: Buildings often have a significant thermal capacitance so that they approach steady state conditions slowly. Southwestern Indians built adobe structures with walls that were 1 to 2 ft thick. In these desert locations, temperatures often fluctuated dramatically between day and night. The thick walls, cooled during the cold nighttime temperatures, heated only slowly during the day. This same energy was lost to the inside and outside air slowly through the night, providing a relatively warm inside temperature. To a lesser extent, homes built today provide a similar lag that may significantly reduce heating/cooling loads.
Solar and background radiation provides a very complex energy input into any structure. o It is both directional and time dependent in nature. During the summer months, the sun is nearly directly overhead through the middle portion of the day, but provides a direct exposure to eastern and western walls during morning and afternoon periods. o It includes both direct and diffuse components. Shade from trees, overhangs or other buildings may eliminate the direct component, but diffuse radiation must still be considered. o Curtains, shutters, awnings, blinds and other window treatments may provide a variety of loads. These loads are not fixed and may vary during the lifetime of the structure.