MECH 8250Building Systems

Winter 2015

1.Lecture: January 12th

Lab: January 13th

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Energy (Heat) Transfer

• Air

• Water

• Steam

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• At standard temperature 20C (70F)• Latent Heat of Water at 0C (32F)

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Thermodynamics

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• 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

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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.

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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.

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Water Heating and Cooling ProcessQ = 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.

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Part 2: Heat Load

1. Conduction

3. Radiation

2. Convection

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HEAT TRANSFER

Conduction Heat transfer

• Fourier’s law of heat conduction

• Empirical statement based on experimental observations and is given by:

• K = Thermal Conductivity

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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.

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Convection Heat transfer

• Convection heat transfer takes place between a surface and a moving fluid, when they are at different temperatures

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Design Conditions - OUTDOORSASHRAE

DBT 99.6%, 99%

-7C -4C18F 24F

ASHRAE

DBT / WBT 0.4%, 1%, 2%

24.4C/18.3C 23.C/17.8C 21.7C/16.7C76F/65F 74F/64F 71F/62F

ASHRAE1. 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 / 19C78.8F / 66.2F

Difference between Critical and Non-critical is 3C (5.6F) = 10% more conservative

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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

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Purpose of Load Estimate

• Load profile over a day

• Peak load (basis for equipment sizing)

• Operation Energy analysis

• HVAC Construction cost

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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

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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 LossesQ sensible = 1.08 * V * (Ti – To)

Q latent = 4840 * V * (Wi – Wo)

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A = AreaU = air to air heat transfer

coefficientTi = Indoor temperatureTo = Outdoor temperature

F = Floor Heat Loss CoefficientP = Perimeter

V = Volumetric air flow rateWi = Humidity ratio of indoor airWo= Humidity ratio of outdoor air

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Heat transfer through a wall

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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 included3. 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.

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