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HVAC. NC STATE UNIVERSITY MECHANICAL & AEROSPACE ENGINEERING MAE - 406 Douglas L. Gunnell, P.E., PEM, CEM Gunnell Engineering Services Clemmons, NC 2010-3. 1. Outline. Definitions Heat Transfer/Heat Generation Heating and Cooling Loads Indoor Air Quality-ASHRAE STD. 62.1
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HVAC NC STATE UNIVERSITY MECHANICAL & AEROSPACE ENGINEERING MAE - 406 Douglas L. Gunnell, P.E., PEM, CEM Gunnell Engineering Services Clemmons, NC 2010-3 1
Outline Definitions Heat Transfer/Heat Generation Heating and Cooling Loads Indoor Air Quality-ASHRAE STD. 62.1 Vapor Compression Refrigeration Cycle Energy Usage Psychrometrics HVAC Systems Air-to-Air Heat Recovery Control Strategy
Air Conditioning Process of treating air so as to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the environmental requirements of the conditioned space Environmental requirements of the conditioned space may be determined by human occupancy as related to comfort and health, a process, or a product
Air Conditioning Processes Heating: transfer of energy to the air in a space Cooling: transfer of energy from the air in a space Humidifying: transfer of water vapor to the air in a space Dehumidifying: removal of water vapor from the air in the space Cleaning: removal of particulate and biological contaminants from the air in a space Air Motion (Circulation): movement of air through the spaces in a building to achieve the proper ventilation and facilitate the energy transfer, humidification (or dehumidification), and cleaning processes described above
Energy The capacity for producing an effect Either stored or transient, and can be transformed from one to another Forms include: thermal (heat), mechanical (work), electrical, chemical
Heat Energy in transit from one mass to another as a result of a temperature difference between two masses A basic law of thermodynamics states that heat always flows from a higher temperature to a lower temperature
Sensible Heat Heat which changes the temperature of a substance without changing its state
Latent Heat Heat which changes the state of a substance without changing its temperature Two familiar examples: latent heat of fusion (changing ice to water) and latent heat of vaporization (changing water to vapor)
British Thermal Unit - BTU A measure of the quantity of heat energy Heat energy in a BTU is that required to raise the temperature of a pound of water 1º Fahrenheit
Heat Energy Flow Rate Rate of heat loss/heat gain associated with buildings Also associated with applied heating and air conditioning equipment Normally stated in the terms BTU/hr
Heat Transfer Movement of heat through surfaces and openings of a building Usually assumed to be steady state (various temperatures throughout a system remain constant with respect to time during heat transmission) Based upon predetermined temperature differences
Heat Loss/ Heat Gain Heat Loss – heat transferred from the interior of a building to its exterior Heat Gain – heat transferred from the exterior to the interior of a building
Heat Transfer Modes Three modes of heat transfer: Conduction, Convection, Radiation Usually all three modes occur simultaneously In some instances, the methods can be separated, but in others, only the combined effect can be determined
Conduction Conduction is the transmission of heat through solids and composite sections such as structural components Conduction does not occur only within one object or substance, it also occurs between different substances that are in contact with one another By building the walls and roofs of a building of materials having known conductive characteristics, the heat flow rate for the building can be controlled
Convection Convection is the transfer of heat due to the movement of a fluid: gases, vapors, and liquids If the fluid moves because of a difference in density resulting from temperature changes, the process is called natural convection or free convection If the fluid is moved by mechanical means (pumps or fans), the process is called forced convection
Radiation • Radiation is the transfer of heat through space by energy carrying electromagnetic waves • Radiant heat passing through air does not warm the air through which it travels • All objects absorb and radiate heat • The amount of radiant heat given off in a specified period of time is dependent on both the temperature as well as the extent and nature of the radiating object
Factors that Determine Building HVAC Energy Use Building configuration and orientation Building envelope construction Interior space arrangement Design temperature and humidity, indoor and outdoor Zoning criteria Equipment application and sizing Control methodologies 21
Building Cooling and Heating Requirements • A function of three heat transfer components: • Heat gains or losses through the building surfaces [walls, fenestration, roof, etc.] • Heat gains from internal heat producing sources [lights, people, appliances, etc.] • Heat gains or losses from infiltration of outdoor air through window and door cracks, floors, walls, etc.
Indoor Design Conditions The primary purpose of the heating and air-conditioning system is to maintain the space in a comfortable and healthy condition This is generally accomplished by maintaining the dry-bulb temperature and the relative humidity within an acceptable range The HVAC Applications Volume of the ASHRAE Handbook gives recommendations for indoor design conditions for specific comfort as well as industrial applications
Indoor Design Conditions…cont’d • ANSI / ASHRAE Standard 55-2004 • “Thermal Environmental Conditions for Human Occupancy” specifies the combinations of indoor thermal environmental factors and personal factors that produce acceptable conditions to a majority of the occupants
ANSI / ASHRAE Std. 55-2004 Acceptable range of operative temperature and humidity
Outdoor Design Conditions Climate Design Information for 4422 locations throughout the world are included on the CD that accompanies the HVAC Fundamentals Volume of the ASHRAE Handbook Appropriate outdoor design conditions can be selected from this ASHRAE document in most applications; however some states dictate the outdoor design conditions in their building codes
Calculation of Overall Heat Transfer Coefficient - U U = overall heat transfer coefficient, BTU/ hr· sf· ºF R = thermal resistance, hr· sf· ºF /BTU R = (0.10)(12.08)+(0.90)(18.70) = 18.04 U = 1 / R = 1 / 18.04 = 0.055
Transmission Heat Loss Through Walls, Roofs, and Glass H = A x U x TD H = heat loss, BTU/hr A = surface area of element, sf U = overall heat transfer coefficient, BTU/ hr· sf· ºF TD = design dry bulb temperature difference between indoors and outdoors, ºF
Transmission Heat Gain Through Walls and Roofs H = A x U x CLTD H = heat gain, BTU/hr A = surface area of element, sf U = overall heat transfer coefficient, BTU/ hr· sf· ºF CLTD = cooling load temperature difference, ºF
Conduction Heat Gains Through Glass Solar Heat Gain Through Glass H = A x SC x SCL H = heat gain, BTU/hr SC = shading coefficient SCL = solar cooling load factor H = A x U x CLTD H = heat gain, BTU/hr A = surface area of element, sf U = overall heat transfer coefficient, BTU/ hr· sf· ºF CLTD = cooling load temperature difference, ºF
Infiltration Heat Gain and Heat Loss The uncontrolled leakage of outdoor air into a building through window and door cracks, floors, walls, etc., as well as the flow of outdoor air into a building through the normal use of exterior doors [Exfiltration is the leakage of indoor air out of the building. The amount of exfiltration equals the amount of infiltration]
Heat Gains from Lights • Each watt of lighting load (including both lamp and ballast) releases 3.413 BTU/hr Heat Gain from Motors • Each brake or net horsepower of motor load divided by the efficiency (including both motor and drive) releases 2545 BTU/hr H = 2545 BTU/hr x Bhp / EffM x EffD H = heat gain, BTU/hr Bhp = brake horsepower • EffM = motor efficiency, decimal fraction, 0 – 1.0 • EffD = drive efficiency, decimal fraction, 0 – 1.0
Heat Gains from Appliances and Equipment Appliances and equipment (including food prep., hospital, lab, office, etc.) normally produce significant sensible heat, and may also produce significant latent heat. To estimate the cooling load, specific heat gain data obtained from the manufacturer is preferred. However, if it is not available, recommended heat gains are published by ASHRAE and other sources. Evaluation of the operating schedule and the load factor for each piece of equipment is essential.
Energy Saving Opportunities Change indoor temperature and/or humidity set-points Improve building thermal envelope Apply additional thermal insulation Improve fenestration Reduce infiltration Improve lighting system efficiency
AHSRAE Standard 62.1Ventilation for Acceptable Indoor Air Quality • Acceptable Indoor Air Quality: Air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction
The Purpose of Standard 62 • The purpose of the Standard, first published in 1973 – “Standards for Natural and Mechanical Ventilation”, has remained consistent: “To specify minimum ventilation rates and other measures intended to provide indoor air quality that is acceptable to human occupants and that minimizes adverse health effects.”
Under Continuous Maintenance… • The standard is updated on a regular bases using ASHRAE’s Continuous Maintenance Procedures • Continuously revised addenda are publicly reviewed and approved by ASHRAE • Published in a Supplement approximately 18 months after each new edition of the Standard OR • A new, complete edition of the Standard is published every three years
Significant Changes to ASHRAE Standard 62 1981 Edition: • Reduced the minimum outdoor air requirements for ventilation • Office – 15 cfm/person to 5 cfm/person 1989 Edition: • Increased minimum outdoor air requirements for ventilation [Response to growing number of buildings with apparent IAQ problems] • Office – 5 cfm/person to 20 cfm/person 2004 Edition: • Changed the ventilation rate procedure to include the summation of two components: the occupant-density related component, and the area related component • Changed the ventilation rates in Table 6-1 to apply to non-smoking spaces
Significant Changed … cont’d 2004 cont’d: • Added classification of air with respect to contaminant and odor intensity, and established guidelines for recirculation 2007 Edition: • Updated information in Table 4-1 – “National primary ambient air quality standards for outdoor air as set by the U.S. Environmental Protection Agency” • Added Section 5.18 – Requirements for buildings containing ETS areas and ETS-free areas (ETS-Environmental Tobacco Smoke)
ASHRAE Standard 62.1 • Two alternative procedures for determining outdoor air intake rates: • Ventilation Rate Procedure • This is a prescriptive procedure in which outdoor air intake rates are determined based on space type/application, occupancy level, and floor area • IAQ Procedure • This is a design procedure in which outdoor air intake rates and other system design parameters are based on an analysis of contaminant concentration targets, and perceived acceptability targets
