Energy transfer and heat load analysis
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Energy transfer in air and water for HVAC systems
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Energy transfer and heat load analysis
- 1. MECH 8250 Building Systems Winter 2015 1. Lecture: January 12th Lab: January 13th 2015-03-31 1Mech8250 - Building Systems
- 2. Energy (Heat) Transfer • Air • Water • Steam Mech8250 - Building Systems • At standard temperature 20C (70F) • Latent Heat of Water at 0C (32F) 2015-03-31 2
- 3. Thermodynamics Mech8250 - Building Systems • 1 psig = 2.31 ft. wg. • 7000 gr = 1.0 lbw AIR Imperial Units SI Units qtotal 4.5 · CFM · ∆h Btu/h 1.22 · L/s. ∆h(W) W qsensible 1.08 · CFM· ∆t (°F) Btu/h 1.22 · L/s · ∆t(°C) W qlatent 0.68 · CFM · W (LBSW/LBSA) Btu/h 3.0 · L/s · ∆w(gw/kgDA) W 4840 · CFM · W (LBSW/LBSA) Btu/h 2500 · L/s · W (kgW/kgdA) W WATER Imperial Units SI Units qtotal 500 · GPM · ∆t(°F) Btu/h 4180· (L/S) · ∆t(°C) W = gpm x (ti‐tf) / 24 Tons Btu/h 2015-03-31 3
- 4. Air Heating and Cooling Process (Sensible Heat Only) Q = 1.08 * CFM * ΔT (BTU/h) Example A room has a heat loss of 10,000 BTU/h. This room is supplied with 95 F heated air to maintain a room temperature of 72 F. Calculate the volume flow rate of the heated air supplied to the room. Mech8250 - Building Systems2015-03-31 4
- 5. Air Heating and Cooling Process (Sensible Heat Only) Q = 1.22 * L/s * ΔT (W) Example An electrical resistance heater is used to heat 250 L/s of supply air to a temperature of 35 C. Calculate the electrical heating capacity for the electrical resistance heater when the heated air is sufficient to maintain a room temperature of 20 C. Mech8250 - Building Systems2015-03-31 5
- 6. Water Heating and Cooling Process Q = 500 * GPM * ΔT Example: Heating water supply flows at 100 gpm through a heating coil with water entering at a temperature of 200F and leaving the coil at a temperature of 180F. Determine the heating coil capacity and also the amount of outdoor air that can be heated from outside temperature at 10F to a room temperature of 70F. Mech8250 - Building Systems2015-03-31 6
- 7. Part 2: Heat Load 1. Conduction 3. Radiation 2. Convection 7Mech8250 - Building Systems2015-03-31 HEAT TRANSFER
- 8. Conduction Heat transfer • Fourier’s law of heat conduction • Empirical statement based on experimental observations and is given by: • K = Thermal Conductivity Mech8250 - Building Systems2015-03-31 8
- 9. Radiation Heat transfer • Transfer does not require a medium for transmission • Energy transfer occurs due to the propagation of electromagnetic waves • A body due to its temperature emits electromagnetic radiation, and it is emitted at all temperatures. • Speed of light (3 x 108 m/s) in a straight line in vacuum. Its speed decreases in a medium but it travels in a straight line in homogenous medium. Mech8250 - Building Systems2015-03-31 9
- 10. Convection Heat transfer • Convection heat transfer takes place between a surface and a moving fluid, when they are at different temperatures Mech8250 - Building Systems2015-03-31 10
- 11. Design Conditions - OUTDOORS ASHRAE DBT 99.6%, 99% -7C -4C 18F 24F ASHRAE DBT / WBT 0.4%, 1%, 2% 24.4C/18.3C 23.C/17.8C 21.7C/16.7C 76F/65F 74F/64F 71F/62F ASHRAE 1. Winters: In cold spells, DBT can drop below the design conditions for a week or more. 2. Summers: The design conditions represent recent conditions on hot, mostly sunny days. BCBC – APPENDIX C – DIVISION B - CLIMATIC AND SEISMIC INFORMATION FOR BUILDING DESIGN IN CANADA 1. Winters: On the basis of average temperatures of January months over past 25 years. 1% =8 hours (Ordinary spaces) ; 2.5% =19 hours (Ordinary spaces) are colder than design 2. Summers: On the basis of average temperatures of July months over past 25 years. 2.5% =19 hours (Ordinary spaces) are hotter than design. Building Code BCBC DBT (January) 1% , 2.5% Vancouver -9C -7C 15.8F 19.4F Building Code BCBC DBT / WBT (July) 2.5% Vancouver 26C / 19C 78.8F / 66.2F Difference between Critical and Non-critical is 3C (5.6F) = 10% more conservative Mech8250 - Building Systems2015-03-31 11
- 12. Heating Load • It is the thermal energy that must be replenished into the space in order to maintain the desired comfort conditions • HVAC systems are used to maintain thermal conditions in comfort range Mech8250 - Building Systems2015-03-31 12
- 13. Purpose of Load Estimate • Load profile over a day • Peak load (basis for equipment sizing) • Operation Energy analysis • HVAC Construction cost Mech8250-BuildingSystems 2015-03-31 13
- 14. Principles of Heating Load Estimate 1. Design conditions – Outdoor & indoor 2. Envelope (Roof, Walls, Windows, Floor) – Conduction – Convection – Radiation 3. Infiltration Loss – Mechanical Ventilation – Cracks and Openings (Air changes) 4. Heat Gains – Not to be accounted for (Why?) – Internal – External or Solar Mech8250 - Building Systems2015-03-31 14
- 15. 1. Enclosure Heat Transfer 1. Transmission Losses 1. Walls, Roof, Windows, Doors etc. Q sensible = A * U * (Ti – To) 2. Floors on Slab Q sensible = F* P * (Ti-To) 2. Infiltration Losses Q sensible = 1.08 * V * (Ti – To) Q latent = 4840 * V * (Wi – Wo) Mech8250 - Building Systems A = Area U = air to air heat transfer coefficient Ti = Indoor temperature To = Outdoor temperature F = Floor Heat Loss Coefficient P = Perimeter V = Volumetric air flow rate Wi = Humidity ratio of indoor air Wo= Humidity ratio of outdoor air 2015-03-31 15
- 16. Heat transfer through a wall Mech8250 - Building Systems2015-03-31 16
- 17. Difference between Winters and Summers 1. Temperatures outside conditioned spaces are generally lower than maintained space temperatures. 2. Credit for solar or internal heat gains is not included 3. Thermal storage effect of building structure or content is ignored. 4. Thermal bridging effects on wall and roof conduction are greater for heating loads than for cooling loads, and greater care must be taken to account for bridging effects on U-factors used in heating load calculations. Mech8250 - Building Systems2015-03-31 17
Hinweis der Redaktion
- Heat transfer is defined as energy-in-transit due to temperature difference. Heat transfer takes place whenever there is a temperature gradient within a system or whenever two systems at different temperatures are brought into thermal contact. Heat, which is energy-in-transit cannot be measured or observed directly, but the effects produced by it can be observed and measured. Since heat transfer involves transfer and/or conversion of energy, all heat transfer processes must obey the first and second laws of thermodynamics. However unlike thermodynamics, heat transfer deals with systems not in thermal equilibrium and using the heat transfer laws it is possible to find the rate at which energy is transferred due to heat transfer. From the engineer’s point of view, estimating the rate of heat transfer is a key requirement. Refrigeration and air conditioning involves heat transfer, hence a good understanding of the fundamentals of heat transfer is a must for a student of refrigeration and air conditioning. This section deals with a brief review of heat transfer relevant to refrigeration and air conditioning. Generally heat transfer takes place in three different modes: conduction, convection and radiation. In most of the engineering problems heat transfer takes place by more than one mode simultaneously, i.e., these heat transfer problems are of multi-mode type.
- Given that Q = 10,000 BTU/h and ΔT = (95 F - 72 F), substituting this into the re-arranged equation gives, CFM = (10,000)/ [1.08 * (95 ‐ 72)] = 403 CFM
- Given that the airflow is 250 L/s and ΔT = (35 - 15, substituting these into the standard formula gives the heating capacity of the electrical heater as, Q = 1.22 * 250 * (35 C ‐ 15C) = 4,575 W
- Given water flow rate of 100 gpm and ΔT = (200F ‐ 180F), Heating Capacity: Q = 500 * 100 gpm * (200 ‐ 180) = 1,000,000 BTU/h The heat output of the heating coil increases the outdoor air flow temperature from a temperature of 10 F to 70F and the amount of outdoor airflow is, Q =1.08 * CFM * ΔT CFM = 15,400 cfm
- Conduction is the process of transferring heat through a solid. Heat travels from higher temperature side (or area) to lower temperature. Convection is the process of transferring heat as the result of the movement of a fluid. Convection often occurs as the result of the natural movement of air caused by temperature (density) differences. Radiation is the process of transferring heat by means of electromagnetic waves, emitted due to the temperature difference between two objects. An interesting thing about radiated heat is that it does not heat the air between the source and the object it contacts; it only heats the object itself.
- Conduction heat transfer takes place whenever a temperature gradient exists in a stationary medium. Conduction is one of the basic modes of heat transfer. On a microscopic level, conduction heat transfer is due to the elastic impact of molecules in fluids, due to molecular vibration and rotation about their lattice positions and due to free electron migration in solids. Fundamental law that governs conduction heat transfer is called Fourier’s law of heat conduction, it is an empirical statement based on experimental observations and is given by this equation The thermal conductivity is equal to the conduction heat transfer per unit cross-sectional area per unit temperature gradient. Thermal conductivity of materials varies significantly. Generally it is very high for pure metals and low for non-metals. Thermal conductivity of solids is generally greater than that of fluids. Thermal conductivity of solids and liquids vary mainly with temperature, while thermal conductivity of gases depend on both temperature and pressure. In refrigeration and air conditioning high thermal conductivity materials are used in the construction of heat exchangers, while low thermal conductivity materials are required for insulating refrigerant pipelines, refrigerated cabinets, building walls etc.
- Radiation is another fundamental mode of heat transfer. Unlike conduction and convection, radiation heat transfer does not require a medium for transmission as energy transfer occurs due to the propagation of electromagnetic waves. A body due to its temperature emits electromagnetic radiation, and it is emitted at all temperatures. It is propagated with the speed of light (3 x 108 m/s) in a straight line in vacuum. Its speed decreases in a medium but it travels in a straight line in homogenous medium.
- Convection heat transfer takes place between a surface and a moving fluid, when they are at different temperatures. Table shows typical order-of-magnitude values of convective heat transfer coefficients for different conditions.
- The heating system is designed with heating equipment that can generate sufficient heat to balance the maximum probable heat loss through the building. Thus, the calculation of heating load is based on estimates of maximum heat losses that normally occur at night. For this reason, the sizing of the heating equipment does not make any allowance for any internal heat sources such as light, people, appliances and decorative fireplace. Moreover, it does not accept credit to reduce the design heating load for any energy saving initiatives such as solar heating, heat storage by the building structure or "turning down the heat", unless the heat source is known to be relatively constant at all hours.