Internal Gains Essay
People and electrical equipment in buildings give off heat. These internal heat gains must be included in energy balances on the building or zone to determine the net heating or cooling load. Heat Gain From People Typical sensible, latent and total heat rates given off by people are shown in the table below (ASHRAE Fundamentals, 2005). The sensible heat gain results from radiation and convection from the human body to the air.
The latent heat gain is the energy required to condense the water vapor given off by human respiration and perspiration. The latent heat gain is the product of the mass of water vapor from humans and the enthalpy of evaporation for water vapor at atmospheric pressure. PersonQtotalQsensibleQlatent Avg. person, at rest350 [Btu/hr]210 [Btu/hr]140 [Btu/hr] 100 W60 W40 W Avg. person, light work640 [Btu/hr]315 [Btu/hr]325 [Btu/hr] 185 W90 W95 W Avg. person, heavy work1600 [Btu/hr]565 [Btu/hr]1035 [Btu/hr] 470 W170 W300 W Heat Gain From Electricity
All electricity consumed in a building is eventually converted into sensible heat. Thus, the heat gain from electricity is equivalent to the electricity use inside a building or zone. The average American house in uses about 11,000 kWh/yr. To gain intuition about how much heat this is, assume the average house uses 8,760 kWh/year. If so, the average rate of heat gain is about: Only the electricity consumed indoors becomes heat gain to the space. Since some electricity use may be for outdoor lighting or be consumed by the air onditioner compressor and condenser fan which are located outside, we might estimate that 75% of electricity is used indoors. If so, then: Thus, the general method for calculating average heat gain from electricity in residences is to find total building electricity use, and multiply by fraction of use that is indoors. For hourly heat gain, weight high use periods heavier and low use periods lighter. Measured power draw from selected household appliances is shown below. Laptop computer25 W Wireless router13 W Printer (inactive)4 W Desktop computer and 17” LCD 85-120 W 7” CRT Television120 W 42” Plasma television200-300 W DVD (inactive)6 W Stereo at low and high volume40 W – 120 W Microwave1,600 W Coffee maker600 W Toaster (2 pieces of toast)690 W Food processor115 W Refrigerator (idle, + door open, + compressor on)7 W, 79 W, 109 W Cellphone charger (inactive, active)0 W, 3 W Iron1,040 W Typical electricity use in commercial buildings is about 1. 5 W/ft2 for lighting and 0. 5 W/ft2 for plug loads such as computers and copiers. All of this is becomes heat gain into the space. Design Heating and Cooling Loads
Heating and cooling equipment must be sized to maintain interior conditions at comfortable levels during most the most extreme weather and occupancy conditions. Thus for “design” purposes, calculate the maximum hourly heating and cooling loads, and select equipment large enough to meet these loads. Design Heating Load Major sensible heat flows into a building during winter are shown below. Using the sign convention defined in the figure above, the net heating load out is: For most extreme case: Therefore, And choose Toa = Toa,min = Toa,heating des Use to size furnaces and boilers.
The method to size heat pumps is explained in the chapter on Heat Pumps. Heating equipment is rated by output capacity. For non-standard sizes, round up. For example, if , then specify a furnace with an output capacity of Design Cooling Load Major sensible heat flows into a building during summer are shown below. Due to energy storage capacity of the ground, Qgrnd is assumed to be negligible. Using the sign convention defined in the figure above, the net cooling load in is: To create the design heat load, consider the most extreme cases when: Then And choose:
Most cooling equipment is rated in “tons” of cooling capacity, where: . Use to specify the size of cooling equipment. For non-standard sizes, round up. For example, if then specify a “3. 0 ton” air conditioner. Design Weather Conditions For system sizing purposes, ASHRAE tabulates the minimum and maximum outdoor air temperatures and coincident outdoor humidity that is likely to occur for hundreds of U. S. cities. In addition, ASHRAE also publishes a method to calculate peak solar radiation on a surface. These tables and methods are included in the chapter on Sequence of Analysis, Energy Balances and Weather Data.