COSC 325

Lecture note #6

Heat loss and heat gain in buildings: Outside air (OA)

1.00 Outside air

• Required to be brought inside a building envelope to continuously replace indoor air that has been contaminated with odors, carbon dioxide, and other air contaminants.
• While its is allowable to filter and recirculate air to remove contaminants under certain circumstances, a minimum air exchange is necessary to decrease carbon dioxide buildup. Such air exchange may take place either through infiltration or ventilation.

1.01 Infiltration

• Leakage around doors and windows and around walls due to outdoor air pressure.
• Occurs in an uncontrolled manner. Winds on the windward side of the building blow outside air into the indoor spaces, displacing conditioned air that leaves through similar opening on the leeward side of the building where low outside air pressure creates suction.
• The outside air that enters the building must be conditioned (heated, cooled, humidified, dehumidified, or cleaned).
• Two methods for calculation of infiltration in buildings: crack length method and air change method.
• Crack length method: Expresses air flow rate per unit length of crack (e.g. a metal framed hinged window may have an infiltration of 0.35 CFM per foot of crack); requires specific information about the dimensions and construction details of doors, windows, or other openings.
• The above method can also be expressed in terms of air flow rate per unit area of door or window (e.g. an operable glass window has a rate of infiltration of 0.5 CFM per square foot of the window area; the rate is about 0.25 CFM per square foot for fixed glass windows).
• The quantity of infiltration depends on the the kind or width of crack and the pressure difference due to wind velocity. It is common to use a wind velocity of 15 mph for normal design conditions.
• Air change method: This method takes into account total replacement of inside air by outdoor in terms of an unit called air change.
  • When construction method and related crack lengths cannot be accurately evaluated, the air change method may be used as an alternative.
• An air change having a value of 1 per hour means that all air in a building is replaced by outside air every hour.
• Formula used for conversion of air change rate to CFM: (Volume of building)*(Air change rate)/60 = CFM.

1.02 Ventilation

• Ventilation refers to air brought into building by choice, intentionally.
• May either be provided mechanically or by natural means.
• The outside air that is allowed to enter the building through ventilation must also be conditioned (heated, cooled, humidified, dehumidified, or cleaned).
• Rate of ventilation determined by the requirements based on number of occupants and their activities.
• Recommended levels calculated either in terms of CFM per person or CFM per square foot.
• Mechanical ventilation is not usually required for small, residential buildings; infiltration (some quantity of air infiltrates, if the building is not hermetically sealed) is sufficient to furnish enough air change for such buildings.

2.00 Calculation of hg/hl due to outside air (OA)

• Heat gain/heat loss due to OA has two components: sensible and latent.
• Formula for calculation of sensible heat due to OA: BTUH = 1.08*CFM*TD
• Formula for calculation of latent heat due to OA: BTUH = 0.68*CFM*GD

3.00 Value of CFM to be used in calculation

• Outside air rate both due to infiltration and ventilation should be found out. The rate which is larger should be used in the calculation of heating/cooling load.

4.00 Some explanations of the constants used in OA heat transfer models

• Specific Heat: Quantity of heat required to raise the temperature of one pound of a material by 1^oF. Specific heat of air is 0.24 BTU/lb/1^oF.
• Why a constant 1.08 in the sensible heat transfer model?
  • At a rate of 1 CF per minute airflow, there are 60 such units in an hour (the hourly rate of airflow is required to be known because heat flow is calculated in terms of per hour). 60 CFM of air = 4.5 lbs of air.
  • Heat required to raise the temperature of 4.5 lbs of air = 0.24*4.5 = 1.08 BTU/lb/1^oF.
• Why a constant of 0.68 in the latent heat transfer model?
  • Heat released in condensation of one pound of water = 1060 BTU.
  • Heat released in condensation of one grain of water = 1060/7000 BTU.
  • Assuming a moisture difference of one grain per pound between inside and outside air, for 60 CFM of air there will be difference of 4.5 grains. Heat released in condensation of that amount of water = (1060/7000)*4.5 = 0.68 BTUH.