Heating and Air Conditioning I

Heating and Air Conditioning I Principles of Heating, Ventilating and Air Conditioning R.H. Howell, H.J. Sauer, and W.J. Coad ASHRAE, 2005 basic textbook/reference material For ME 421 John P. Renie Adjunct Professor – Spring 2009

Chapter 5 – Design Conditions • This chapter covers the fundamental elements that accompany the load calculations for sizing heating and cooling systems. This includes the estimation of the outside air quantities and the evaluation of the overall heat transfer coefficient for building components. • Basic components of heating and cooling loads • Transmission • Solar Radiation • Lights • Power • Infiltration (uncontrolled) • Fresh Air (controlled) • People • Appliances • Material in and out • First four terms include Q • Last five includes Q and mwater

Chapter 5 – Design Conditions • General Considerations • Knowledge of thermal insulation and thermal behavior of materials • Flowing fluids – carry heat and moisture • Heating and cooling include simultaneous transfer of heat and mass • Humidification and dehumidification depends on ventilation and other sources of moisture gain and loss. • Outdoor Air Load Components • Outdoor air flows through building – dilute and remove indoor air contaminants – significant portion of total space load • Natural infiltration in addition to intentional mechanical ventilation • Ventilation (both natural and mechanical) versus infiltration • Air leakage into and out (exfiltration) due to pressure difference • Modern building usually over pressurized • Modern building – not desirable to have open windows • Residence building – reliant on infiltration and natural ventilation

Chapter 5 – Design Conditions • Outdoor Air Load Components • Small exhaust fans – code (condensation?) • ASHRAE Standard 62 – guidance on ventilation and indoor air quality • Ventilation Rate Procedure • Control of six pollutants • Outdoor air supply > minimums (5 cfm per person and 0.06 cfm/ft2) • Indoor Air Quality Procedure • Nine pollutants indoor maintained below standard level • Subjective (0.5 AC/hr) – local codes • Terminolgy • Air handling unit (AHU) – Figure 5-2 • Return air (ra) • Exhaust air (ea) • Recirculated air (ca) • Outside air (oa) – not “fresh” – outside air fraction Xoa or percent • Mixed air (ma) • Supply air (sa) • Bypass air (ba)

Chapter 5 – Design Conditions

Chapter 5 – Design Conditions • Outdoor Air Load Components. • Terminolgy • Makeup air unit (MAU) – all air is outside • Outside air • Large part of the total space conditioning (heating, cooling, humidification, and dehumidification) – 20 to 40% • Sensible heating – qs = Q r cpDt • Latent heating – qL = Q r hfgDW • Dehumidification in summer • Humidification in winter • Infiltration • Air leakage through cracks and interstices around windows and doors, and through the floors and walls – depends on construction, building condition • Pressure difference between indoor and outdoor – caused by wind or differences in density – chimney or stack effect – also differences in temperature

Chapter 5 – Design Conditions • Outdoor Air Load Components. • Estimation of the air infiltration • Crack method – measured leakage characteristics of the building components • Air change method (simplicity) • Assumption of the number of air changes per hour (ACH) that a space experiences • Q = ACH x VOL/60 where VOL is gross volume of space • Table 5-1 – Change Rates as Function of Airtightness • Outdoor design temperature • Tight/Medium/Loose • Figure 5-3 Building Pressure due to Wind Effects • Impossible to accurately predict infiltration from theory alone because of the many unknowns. However, semi-empirical expressions can be used to estimate infiltration rate. • Leakage function – infiltration cause by pressure difference • Q = CD A (2 Dp/r)n • Where CD is discharge coefficient (geometry and Reynolds number) • Dp = Dps + Dpw + Dpp (stack, wind, building pressurization)

Chapter 5 – Design Conditions • Outdoor Air Load Components

Chapter 5 – Design Conditions • Outdoor Air Load Components. • Estimation of the air infiltration • Stack effect – occurs when air densities are different on the inside and outside of the building – density decreases with increasing temperature • Winter – lower outside temperature at higher pressure than inside • Summer – cooler air inside – air infiltrates at top and flows downward • Neutral axis • Dps = 0.52 pb h [(1/To) – (1/Ti)] where pb is barometric pressure • Wind velocity effect (velocity pressure) • Dpw = 0.5 Cpr Vw2 • Figure 5-5 gives average pressure coefficients for tall buildings • Building pressurization – depends on design and operation of the HVAC system. • Positive pressurization causes less infiltration from stack and wind effects • Looking at crack coefficient equation - Q = K A (Dp)n • With A is wall area and K is leakage coefficient • Table 5-2 Curtain Wall Leakage Coefficients – Figure 5-6 • Tight (K = 0.22), Average (K = 0.66), and Loose (K = 1.30)

Chapter 5 – Design Conditions • Outdoor Air Load Components. • Estimation of the air infiltration • Typical infiltrations values for North America residential housing – varies by order of magnitude • Figure 5-7 Histogram of Infiltration Values – New Construction • Figure 5-8 Histogram of Infiltration Values – Low-Income Construction • Commercial building envelops are thought to be nearly airtight. • At 0.30 in water – 0.1 to 0.6 ACH • Regression equation for rate (I = ACH) based on Dt and V • I = K1 + K2Dt + K3 V • Not appropriate for simulations • Multi-cell modeling – complexity • Residential – single zone effective leakage area at 0.016 inch water • Q = AL (CSDt + CW V2)0.5 • Cw based on shielding class (Table 5-4) – wind coefficient – Table 5-6 • Cs values based on number of stories (Table 5-5) – stack coefficient • Commercial doors – different characteristics – swinging door – Figure 5-9 • Figure 5-10 through 5-14 – different styles, with traffic, etc.

Chapter 5 – Design Conditions • Outdoor Air Load Components. • Ventilation Air • Acceptable Indoor Air Quality (IAQ) – debated for years • Control of moisture, carbon dioxide, odors, tobacco smoke • Additional pollutants not generated by occupants – Table 5-7 • Ventilation rate procedure – meet outside air quality standards – Table 5-8 • Ventilation effectiveness – ability to remove internally generated pollutants • See Table 5-9 Minimum Ventilation Rates in Breathing Zone • Indoor air quality procedure – amount of outdoor air ay be reduced by recirculating air which offending contaminates have been removed or converted to less objectionable forms • Figure 5-15 • Nomenclature (C = contaminant concentration, Fr + reduction factor, N = contaminate generation rate, R = recirculation rate) • Filter placement • Variable air volume (VAV) systems • See Figure 5-16 for required outdoor air with recirculation and filtration. • See Example 5-3 – Chicago fast-food cafeteria

Chapter 5 – Design Conditions • Outdoor Air Load Components - Example

Chapter 5 – Design Conditions • Heat Transfer Coefficients • Modes of heat transfer (driven by DT) • Conduction – transfer of heat through molecular motion • Convection – enhancement of conduction through motion • Radiation – electromagnetic radiation between surfaces at DT • U-Factor or R-value – resistance to flow of energy • Thermal resistance of building materials (Table 5-11) • Apparent thermal conductivity

Chapter 5 – Design Conditions • Heat Transfer Coefficients • Calculation of U requires • Apparent thermal conductivity and thickness of material • Thermal conductance of non-homogeneous material • Surface conductance of surfaces • Conductance of air spaces • Surface Conductance • Combined effects of radiation, convection, and conduction • Surface properties (reflectivity, emissivity, roughness) • Geometry, viewing angles, • Flow behavior, etc. • Other factors • Workmanship • Air-tightness • Irregular areas, shading • Shrinkage, settling, compression,etc.

Chapter 5 – Design Conditions • R-Values - materials

Chapter 5 – Design Conditions • Surface Conductances and Resistances

Chapter 5 – Design Conditions • Emittance Values of Surfaces and Airspaces

Chapter 5 – Design Conditions • Thermal Resistances of Plane Airspaces

Chapter 5 – Design Conditions • Thermal Properties of Common Building Materials (pgs. 123-127)

Chapter 5 – Design Conditions • Determining U-factors • Total resistance (both conductances and air film) – U = 1/Rtot • Parallel heat paths • Add the U factors (on a percentage basis) • Unequal areas (pitched roof and ceiling areas) • Windows and doors - Tables 5-16 and 5-17

Chapter 5 – Design Conditions • Determining U-factors

Chapter 5 – Design Conditions • Determining U-factors – fenestration products

Chapter 5 – Design Conditions • Determining U-factors - Doors

Chapter 5 – Design Conditions • The Overall Thermal Transmittance

Chapter 5 – Design Conditions • Calculating Surface Temperatures • Voltage divider network due to resistances – driving DT • Account for temperature effect on the resistance of material • Look at example of structure wall …

Chapter 5 – Design Conditions • Calculating Surface Temperatures

Chapter 5 – Design Conditions • Example 5-3 – Determining Uoverall

Chapter 5 – Design Conditions • Example 5-5 – Determining Uoverall

Heating and Air Conditioning I