Radiation does not head the adjacent air it only heats the objects that the electromagnetic waves impact. Finally, the warm casing surface transfers heat to objects in room air by electromagnetic energy or radiation (figure 1). The buoyant (less dense) air within the casing then rises and mixes with the cooler room air by the process of convection. The fins of baseboard then warm the air contained within the casing. Heat is transferred from the warm inner tube to the cooler outer fins by conduction. An example of a device that exhibits all three methods of heat transfer is a baseboard convector. Radiation does not require the presence of matter to propagate. It represents a conversion of thermal energy into electromagnetic energy. Finally, radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. Conversely, conduction is the transfer of energy between adjacent molecules of a solid. Convection is the transfer of energy between two fluids.
Heat energy can be transferred from one substance to another substance by one of three processes convection, conduction and radiation. Thus heat always flow from a high temperature source to a low temperature source much the same way a moving object transfer s energy to a stationary object. When an object in motion impacts with an object that is stationary, it imparts some of its energy to the stationary object (via movement and plastic deformation). Heat always flows from a higher temperature source to a lower temperature source. This is the first law of thermodynamics and is known as the conservation of energy. Energy can be transferred from one substance to another substance, but the energy is maintained. Heat can be transferred from one substance to another substance Heat energy cannot be destroyed. Heat always flows from a higher temperature source to a lower temperature source 3. Fundamentals of Heat Transfer The three primary rules of heat transfer are: 1. If the principles are understood for cooling load estimation, heating load estimation can easily be inferred. In addition, this clinic does not cover the process of determining the heating load.
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It is not intended to teach all the details or latest computerized techniques of how to calculate these loads. This includes the variables that affect each of these components, and simple methods used to estimate these load components. It is intended to introduce the concepts of estimating building cooling and is limited to introducing the components that make up the load on a building. This particular clinic introduces the reader to cooling load estimation. An accurate load provides us with the information required to size our plant, determine the fraction of outside air, and most importantly, how to maintain occupant comfort. Without an accurate load, we couldn t perform the psychrometrics required to determine the cooling airflow or the refrigeration load. 33ģ Introduction Cooling load estimation is the foundation for which everything is based in HVAC. 5 Example Example Weather Design Conditions Design Conditions For Reno, NV (ASHRAE 2005 Fundamentals Rates Of Heat Gain from Occupants (ASHRAE 1997 Fundamentals, Chapter 28, Table 3) Heat Gain for Appliances (ASHRAE 1997 Fundamentals, Chapter 28, Table 5 & 6) Thermal Properties Of Layers (ASHRAE 1997 Fundamentals, Chapter 28, Table 11) Roofs Roof Group Numbers & CLTD (ASHRAE 1997 Fundamentals, Chapter 28, Table 30 & 31) Walls Wall Types Mass Located Inside Wall (ASHRAE 1997 Fundamentals, Chapter 28, Table 33A) Wall Types Mass Evenly Distributed (ASHRAE 1997 Fundamentals, Chapter 28, Table 33B) Wall Types Mass Evenly Distributed (ASHRAE 1997 Fundamentals, Chapter 28, Table 33B) Wall CLTD (ASHRAE 1997 Fundamentals, Chapter 28, Table 33B) Wall Types Mass Located Inside Wall (ASHRAE 1997 Fundamentals, Chapter 28, Table 33A) Glass/Fenestration U-Factors for Various Fenestration (ASHRAE 2005 Fundamentals, Chapter 29, Table 4) Shading Coefficient (ASHRAE 1997 Fundamentals, Chapter 29, Table 11) CLTD for Conduction through Glass (ASHRAE 1997 Fundamentals, Chapter 28, Table 34) Zone Types for Use with SCL (ASHRAE 1997 Fundamentals, Chapter 28, Table 35) Zone Types for Use with SCL (ASHRAE 1997 Fundamentals, Chapter 28, Table 35) Infiltration Infiltration Models (US Department Of Energy, Table 3, Table 4, Table 5).
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