Cooling Load Calculations and Principles in HVAC – Part 3

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Design Information

To calculate the space cooling load, detailed building information, location, site and weather data, internal design information and operating schedules are required. Information regarding the outdoor design conditions and desired indoor conditions are the starting point for the load calculation and is discussed below.

Outdoor Design Weather Conditions

ASHRAE Handbook 1993 Fundamentals (Chapter 26) list tables of climate conditions for the US, Canada and other International locations: In these tables:

The information provided in table 1a, 2a and 3a are for heating design conditions that include:

  1. Dry bulb temperatures corresponding to 99.6% and 99% annual cumulative frequency of occurrence,
  2. Wind speeds corresponding to 1%, 2.5% and 5% annual cumulative frequency of occurrence,
  3. Wind direction most frequently occurring with 99.6% and 0.4% dry-bulb temperatures and
  4. Average of annual extreme maximum and minimum dry-bulb temperatures and standard deviations.

The information provided in table 1b, 2b and 3b are for cooling and humidity control conditions that include:

  1. Dry bulb temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and the mean coincident wet-bulb temperature (warm). These conditions appear in sets of dry bulb (DB) temperature and the mean coincident wet bulb (MWB) temperature since both values are needed to determine the sensible and latent (dehumidification) loads in the cooling mode.
  2. Wet-bulb temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and the mean coincident dry-bulb temperature
  3. Dew-point temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and the mean coincident dry-bulb temperature and humidity ratio (calculated for the dew-point temperature at the standard atmospheric pressure at the elevation of the station).
  4. Mean daily range (DR) of the dry bulb temperature, which is the mean of the temperature difference between daily maximum and minimum temperatures for the warmest month (highest average dry-bulb temperature). These are used to correct CLTD values.

In choosing the HVAC outdoor design conditions, it is neither economical nor practical to design equipment either for the annual hottest temperature or annual minimum temperature, since the peak or the lowest temperatures may occur only for a few hours over the span of several years. Economically speaking short duration peaks above the system capacity might be tolerated at significant reductions in first cost; this is a simple risk – benefit decision for each building design.

Therefore, as a practice, the ‘design temperature and humidity’ conditions are based on frequency of occurrence. The summer design conditions have been presented for annual percentile values of 0.4, 1 and 2% and winter month conditions are based on annual percentiles of 99.6 and 99%. The term “design condition” refers to the %age of time in a year (8760 hours), the values of dry-bulb, dew-point and wet-bulb temperature exceed by the indicated percentage. The 0.4%, 1.0%, 2.0% and 5.0% values are exceeded on average by 35, 88, 175 and 438 hours.

The 99% and 99.6% cold values are defined in the same way but are viewed as the values for which the corresponding weather element are less than the design condition 88 and 35 hours, respectively. 99.6% value suggests that the outdoor temperature is equal to or lower than design data 0.4% of the time.

Design condition is used to calculate maximum heat gain and maximum heat loss of the building. For comfort cooling, use of the 2.5% occurrence and for heating use of 99% values is recommended. The 2.5% design condition means that the outside summer temperature and coincident air moisture content will be exceeded only 2.5% of hours from June to September or 73 out of 2928 hours (of these summer months) i.e. 2.5% of the time in a year, the outdoor air temperature will be above the design condition.

Note, in energy use calculations, hour-by-hour outdoor climate data of a design day should be adopted instead of summer and winter design values.

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Indoor Design Conditions and Thermal Comfort

The indoor design conditions are directly related to human comfort. Current comfort standards, ASHRAE Standard 55-1992 and ISO Standard 7730, specify a “comfort zone,” representing the optimal range and combinations of thermal factors (air temperature, radiant temperature, air velocity, humidity) and personal factors (clothing and activity level) with which at least 80% of the building occupants are expected to express satisfaction. The environmental factors that affect the thermal comfort of the occupants in an air-conditioned space are mainly:

  1. Metabolic rate, expressed in met (1 met = 18.46 Btu/hr.ft²) determines the amount of heat that must be released from the human body and it depends mainly on the intensity of the physical activity.
  2. Indoor air temperature (Tr) and mean radiant temperature (Trad), both in °F. Tr affects both the sensible heat exchange and evaporative losses, and Trad affects only sensible heat exchange.
  3. Relative humidity of the indoor air in %, which is the primary factor that influences evaporative heat loss.
  4. Air velocity of the indoor air in fpm, which affects the heat transfer coefficients and therefore the sensible heat exchange and evaporative loss.
  5. Clothing insulation in clo (1 clo = 0.88 h.ft².°F/Btu), affects the sensible heat loss. Clothing insulation for occupants is typically 0.6 clo in summer and 0.8 to 1.2 clo in winter.

For comfort air-conditioning systems, according to ANSI/ASHRAE Standard 55-1992 and ASHRAE/IES Standard 90.1-1989, the following indoor design temperatures and air velocities apply for conditioned spaces where the occupant’s activity level is 1.2 met, indoor space relative humidity is 50% (in summer only), and Tr = Trad:

DESIGN INFORMATION

If a suit jacket is the clothing during summer for occupants, the summer indoor design temperature should be dropped to 74 to 75°F.

The recommended indoor relative humidity, in %, is:

DESIGN INFORMATION

The Psychrometric chapter of the Fundamentals Handbook(Chapter 6, 2001) provides more details on this aspect. The load calculations are usually based at 75°F dry bulb temperatures & 50% relative humidity.

Indoor Air Quality and Outdoor Air Requirements

According to the National Institute for Occupational Safety and Health (NIOSH), 1989, the causes of indoor air quality complaints in buildings are inadequate outdoor ventilation air. There are three basic means of improving indoor air quality: (1) eliminate or reduce the source of air pollution, (2) enhance the efficiency of air filtration, and (3) increase the ventilation(outdoor) air intake.

Abridged outdoor air requirements listed in ANSI/ASHRAE Standard 62-1989 are as follows:

DESIGN INFORMATION

These ventilation requirements are based on the analysis of dilution of CO2 as the representative human bio-effluent. As per ASHRAE standard 62-1999, comfort criteria with respect to human bio-effluents is likely to be satisfied, if the indoor carbon dioxide concentrations remain within 700 ppm above the outdoor air carbon dioxide concentration.

Refer to ANSI/ASHRAE Standard 62-1999 for details.

Building Pressurization

The outdoor air requirements are sometimes governed by the building pressurization needs. Most air-conditioning systems are designed to maintain a slightly higher pressure than the surroundings, a positive pressure, to prevent or reduce infiltration and untreated air entering the space directly. For laboratories, restrooms, or workshops where toxic, hazardous, or objectionable gases or contaminants are produced, a slightly lower pressure than the surroundings, a negative pressure, should be maintained to prevent or reduce the diffusion of these contaminants to the surrounding area.

For comfort air-conditioning systems, the recommended pressure differential between the indoor and outdoor air is 0.02 to 0.05 inch-WG. WG indicates the pressure at the bottom of a top-opened water column of specific inches of height; 1 in -WG = 0.03612 psig.

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Building Characteristics

To calculate space heat gain, the following information on building envelope is required:

  1. Architectural plans, sections and elevations – for estimating building dimensions/area/volume
  2. Building orientation (N, S, E, W, NE, SE, SW, NW, etc), location etc
  3. External/Internal shading, ground reflectance etc.
  4. Materials of construction for external walls, roofs, windows, doors, internal walls, partitions, ceiling, insulating materials and thicknesses, external wall and roof colors – select and/or compute U-values for walls, roof, windows, doors, partitions, etc. Check if the structure is insulated and/or exposed to high wind.
  5. Amount of glass, type and shading on windows
Operating Schedules

Obtain the schedule of occupants, lighting, equipment, appliances, and processes that contribute to the internal loads and determine whether air conditioning equipment will be operated continuously or intermittently (such as, shut down during off periods, night set-back, and weekend shutdown). Gather the following information:

  • Lighting requirements, types of lighting fixtures
  • Appliances requirements such as computers, printers, fax machines, water coolers, refrigerators, microwave, miscellaneous electrical panels, cables etc
  • Heat released by the HVAC equipment.
  • Number of occupants, time of building occupancy and type of building occupancy
COOLING LOAD METHODOLOGY – CONSIDERATIONS & ASSUMPTIONS

Design cooling load takes into account all the loads experienced by a building under a specific set of assumed conditions. The assumptions behind design cooling load are as follows:

  1. Weather conditions are selected from a long-term statistical database. The conditions will not necessary represent any actual year, but are representative of the location of the building. ASHRAE has tabulated such data.
  2. The solar loads on the building are assumed to be those that would occur on a clear day in the month chosen for the calculations.
  3. The building occupancy is assumed to be at full design capacity.
  4. The ventilation rates are either assumed on air changes or based on maximum occupancy expected.
  5. All building equipment and appliances are considered to be operating at a reasonably representative capacity.
  6. Lights and appliances are assumed to be operating as expected for a typical day of design occupancy.
  7. Latent as well as sensible loads are considered.
  8. Heat flow is analyzed assuming dynamic conditions, which means that heat storage in building envelope and interior materials is considered.
  9. The latent heat gain is assumed to become cooling load instantly, whereas the sensible heat gain is partially delayed depending on the characteristics of the conditioned space. According to the ASHRAE regulations, the sensible heat gain from people is assumed 30% convection (instant cooling load) and 70% radiative (delayed portion).
  10. Peak load calculations evaluate the maximum load to size and select the refrigeration equipment. The energy analysis program compares the total energy use in a certain period with various alternatives in order to determine the optimum one.
  11. Space (zone) cooling load is used to calculate the supply volume flow rate and to determine the size of the air system, ducts, terminals, and diffusers. The coil load is used to determine the size of the cooling coil and the refrigeration system. Space cooling load is a component of the cooling coil load.
  12. The heat transfer due to ventilation is not a load on the building but a load on the system.
Thermal Zoning

Thermal zoning is a method of designing and controlling the HVAC system so that occupied areas can be maintained at a different temperature than unoccupied areas using independent setback thermostats. A zone is defined as a space or group of spaces in a building having similar heating and cooling requirements throughout its occupied area so that comfort conditions may be controlled by a single thermostat.

When doing the cooling load calculations, always divide the building into zones. Always estimate the building peak load and individual zones airflow rate. The building peak load is used for sizing the refrigeration capacity and the individual zone loads are helpful in estimating the airflow rates (air-handling unit capacity).

In practice the corner rooms and the perimetric spaces of the building have variations in load as compared to the interior core areas. The following facts may be noted:

  • The buildings are usually divided into two major zones.
    • Exterior Zone: The area inward from the outside wall (usually 12 to 18 feet, if rooms do not line the outside wall). The exterior zone is directly affected by outdoor conditions during summer and winter.
    • Interior Zone: The area contained by the external zone. The interior zone is only slightly affected by outdoor conditions and usually has a uniform cooling.
  • Single-zone models shall be limited to open floor plans with perimeter walls not exceeding 40 feet in length.
  • For large building footprints, assume a minimum of five zones per floor: one zone for each exposure (north, south, east & west) and an interior zone.

 

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