Cooling/Heating Load Computations The most complex task in HVAC design. Involving many physical phenomena such as heat conduction, fluid convection, solar radiation/thermal radiation, heat and moisture dissipation from human body and equipment, and complex building structures. The load computation is essential for comfort level of occupants and sizing/purchasing HVAC equipment. Normally, the load calculation is undertaking using a sophisticated computer program.

Human body Cooling Mechanisms/the needs for Air Conditioning Heat dissipation mechanisms from a human body: The average temperature for healthy adults is around 98.2 °F or 36.8 °C (Homeostasis). Derived internally via metabolism , a certain amount of heat must be dissipated for a person's survival (the needs for cooling).   Body heat generation rate + heat received from external heat sources = dissipation through sensible heat convection + dissipation through latent heat (skin evaporation/sweating/breathing) + radiation heat exchange with the surroundings. On the other hand, if the cooling by the surroundings exceeds the heat generation, the desired temperature range may be unable to maintain (the needs for heating). Thermal management of human body is directly related to the surrounding temperature and moisture level (relative humidity). To maintain acceptable temperature/moisture level, the heat gained by the conditioned space must be removed.

Heat Gains (Cooling load is directly related to the heat gain)

 Heat Gains

Internal Heat gain -People Adjusted heat gain is a nominal value, based on the normal percentage of men, women, and children for the application listed.

Internal Heat Gain-Lights/Miscellaneous Equipment Miscellaneous Equipment include motors, appliances, kitchen installation, lab facilities, office equipment, etc.

External Heat Gain-Radiation Solar radiation (short wavelength radiation):

Solar-Irradiation on a Surface (walls/roofs) (Google)

Solar-radiation through windows (Google) The Solar Heat Gain Coefficient for common uncoated window glass is .84 BUT this does not mean 84 % of the energy from the sunlight stays inside the sunspace or solar collector. 16 % of the IR heat radiation will pass back out so only 44% of the incident solar radiation or INSOLATION is trapped by a greenhouse. Heat is also lost by conductance through the glass. Common thermopane glass has an R value of 2. Argon filled thermopane has an R rating of 3. Evacuated thermopane may have R values as high as 12 BUT maintaining a leek proof vacuum inside thermopane is difficult and expensive.

Radiation Heat Transfer between surfaces (Google)

Thermal Radiation (long wavelength)

Thermal Radiation (long wavelength)

Thermal Radiation (long wavelength)

Convection Heat Transfer between a surface and a fluid (Google)

Convection Heat Transfer associated with buildings (Google)

 Exterior Convection

 Exterior Convection

The Cooling Load is Transient Cooling load is the rate at which energy must be removed from a space to maintain the temperature and humidity at the design values. Cooling load is determined by internal and external heat gains as well as structure variable heat storage, all of which are transient in nature Instantaneous heat gain is the heat received by the space being conditioned. Instantaneous cooling load is the heat gain by the air in the space

 The Cooling Load This figure is for solar heat gain (TSHG: total solar heat gain) and the associated cooling load. A heavy weight (HW) construction has the benefit of reducing the peak cooling load, which reduces the size of required coolers. Cooling load attributed to lights. Actual cooling load will approach the heat gain if lights remain on for a sufficiently long time.

Heat Balance Method for Cooling load computation Building Walls/Roofs

Heat Balance on the jth exterior surface at a given time

Heat Balance on jth Interior surface at a given time

Heat Balance on the Zone Air

Transient Conduction Heat Transfer through a Solid Wall The use of Response Factors or Conduction Heat Transfer Functions (CTFs) for closed form solutions

Transient Conduction Heat Transfer through a Solid Wall (continued) The use of Response Factors or Conduction Heat Transfer Functions (CTFs) For transient heat transfer, the heat flux at a given time and location is related to the temperatures or heat fluxes at earlier times.

 Example 8-1

 Example 8-2

 Example 8-2

 Example 8-3

Fenestration-Transmitted Solar Radiation

Fenestration-Simplified Model Transmitted direct radiation is incident on the floor and absorbed in proportion to the floor solar absorptance. The reflected portion will be assumed to be diffuse reflected and uniformly absorbed by all surfaces. All transmitted diffuse radiation is uniformly absorbed by all of the zone surfaces.

 Example 8-4

Interior Surface Heat Balance (Opaque surface) The thermal radiation includes surface-to-surface radiation and the radiation portion of the internal heat gains.

  Surface-to-surface radiation-Mean radiant temperature (MRT) method-a simplified method Radiation heat transfer between walls, ceiling, floor, and furnishings (e.g., desks, chairs, tables, shelves). Furnishings are usually lumped into a single surface, called internal mass. Radiation from equipment, lights and people are treated separately.

  Surface-to-surface radiation-Mean radiant temperature (MRT) method-a simplified method

Mean radiant temperature (MRT) method

Mean radiant temperature (MRT) method

Internal heat gain radiation

 Example 8-5

 Example 8-5

 Example 8-6

 Example 8-8

 Example 8-8

 Example 8-9

 Example 8-10

 Example 8-11

 Design Condition Indoor design conditions: For average jobs in the United States and Canada, a condition of 75 F or 24 C dry bulb temperature and relative humidity of 50% is typical when activity and dress of the occupants are light. Outdoor design temperatures would depend on the selection of the high end temperatures that equal or exceed a percentage of the hours that occurred during a year. A smaller percentage may result in a greater excess cooling capacity to be installed. (Table B-1 (1a: English/1b: SI): MWS = mean coincident wind speed, MWD = mean coincident wind direction, MWB = mean coincident wet bulb temperature, MDB = mean coincident dry bulb temperature, HR: humidity ratio, in grains of water per pound of air (7000 grains equal 1 pound), Range = daily range of db temperature (the difference between the average maximum and average minimum for the warmest month).

 Design Conditions

HvacLoadExplorer Software Based on the heat balance method (HBM). It allows a user to run a cooling or heating load calculation for an entire building to determine the cooling or heating loads and airflow rate of all the rooms in a building or zone. It allows a user to calculate the conduction transfer function coefficients, response factors, radiant time series factors. For purposes of load calculations, a building may be thought of as being organized in a hierarchical fashion. That is, a building is made up of zones, zones are made up of rooms, and rooms are made up of walls, roofs, floors, and other heat gain elements such as people, lighting, equipment, and infiltration. Zones are collections of one or more rooms, all controlled to the same air temperature to allow the user to compute total loads for collections of rooms. A user may wish to include all rooms served by a particular cooling coil in a single zone so that the peak load on the coil can be readily determined. Double-clicking on a zone will show all of the rooms in the zone; double-clicking on a room will show all of the heat gain elements in the room.

 Quick Start Guide First, to start describing a new building, go to the File menu and select “New”. This will ask you to specify a file name. After this step, zones may be added to the building, rooms may be added to the zone, and heat gain elements may be added to the room, with the “Add Node” button shown in Figure 1.1. Jumping ahead (assuming many of the additional steps are intuitive) users need to be aware that when they specify a wall, roof, or floor, it is important to specify an external boundary condition, as shown in Fig. 1-2. “TOS” should be used for exterior surfaces; “TA” for interior surfaces SW absorptivity = short wave absorptivity, related to solar irradiation LW emissivity = long wave emissivity, related to thermal radiation

 Main view

 Tree view First, to start describing a new building, go to the File menu and select “New”. This will ask you to specify a file name. After this step, zones may be added to the building, rooms may be added to the zone, and heat gain elements may be added to the room, with the “Add Node” button shown in Figure 1.1. Jumping ahead (assuming many of the additional steps are intuitive) users need to be aware that when they specify a wall, roof, or floor, it is important to specify an external boundary condition, as shown in Fig. 1-2. “TOS” should be used for exterior surfaces; “TA” for interior surfaces SW absorptivity = short wave absorptivity, related to solar irradiation LW emissivity = long wave emissivity, related to thermal radiation

 List view

Setting boundary conditions for wall

Setting structures of the wall