Major Factors in HVAC Design

This article presents the necessary concepts for management of heat energy and aims at development of a better understanding of its effects on human comfort. The concepts must be well understood if they are to be applied successfully to modification of the environment in building interiors, computer facilities, and manufacturing processes.

Significance of Design Criteria

Achievement of the desired performance of any HVAC system, whether designed for human comfort or industrial production or industrial process requirements, is significantly related to the development of appropriate and accurate design criteria.
Some of the more common items that are generally considered are as follows:
1. Outside design temperatures:
Winter and summer
Dry bulb (DB), wet bulb (WB)
2. Inside design temperatures:
Winter: heating F DB and relative humidity
Summer: cooling F DB and relative humidity
3. Filtration efficiency of supply air
4. Ventilation requirements
5. Exhaust requirements
6. Humidification
7. Dehumidification
8. Air-change rates
9. Positive-pressure areas
10. Negative-pressure areas
11. Balanced-pressure areas
12. Contaminated exhausts
13. Chemical exhausts and fume hoods
14. Energy conservation devices
15. Economizer system
16. Enthalpy control system
17. Infiltration
18. Exfiltration
19. Controls

Design Criteria Accuracy

Some engineers apply much effort to determination of design conditions with great accuracy. This is usually not necessary, because of the great number of variables involved in the design process. Strict design criteria will increase the cost of the necessary machinery for such optimum conditions and may be unnecessary. It is generally recognized that it is impossible to provide a specific indoor condition that will satisfy every occupant at all times. Hence, HVAC engineers tend to be practical in their designs and accept the fact that the occupants will adapt to minor variations from ideal conditions. Engineers also know that human comfort depends on the type and quantity of clothing worn by the occupants, the types of activities performed, environmental conditions, duration of occupancy, ventilation air, and closeness of and number of people within the conditioned space and recognize that these conditions are usually unpredictable.

Outline of Design Procedure

Design of an HVAC system is not a simple task. The procedure varies considerably  from one application or project to another, and important considerations for one project may have little impact on another. But for all projects, to some extent, the following major steps have to be taken:
1. Determine all applicable design conditions, such as inside and outside temperature and humidity conditions for winter and summer conditions, including prevailing winds and speeds.
2. Determine all particular and peculiar interior space conditions that will be maintained.
3. Estimate, for every space, heating or cooling loads from adjacent unheated or uncooled spaces.
4. Carefully check architectural drawings for all building materials used for walls, roofs, floors, ceilings, doors, etc., and determine the necessary thermal coefficients for each.
5. Establish values for air infiltration and exfiltration quantities, for use in determining heat losses and heat gains.
6. Determine ventilation quantities and corresponding loads for heat losses and heat gains.
7. Determine heat or cooling loads due to internal machinery, equipment, lights, motors, etc.
8. Include allowance for effects of solar load.
9. Total the heat losses requiring heating of spaces and heat gains requiring cooling of spaces, to determine equipment capacities.
10. Determine system type and control method to be applied.

Temperatures Determined by Heat Balances

In cold weather, comfortable indoor temperatures may have to be maintained by a heating device. It should provide heat to the space at the same rate as the space is losing heat. Similarly, when cooling is required, heat should be removed from the space at the same rate that it is gaining heat. In each case, there must be a heatbalance between heat in and heat out when heating and the reverse in cooling.
Comfortable inside conditions can only be maintained if this heat balance can be controlled or maintained.
The rate at which heat is gained or lost is a function of the difference between the inside air temperature to be maintained and the outside air temperature. Such temperatures must be established for design purposes in order to properly size and select HVAC equipment that will maintain the desired design conditions. Many other conditions that also affect the flow of heat in and out of buildings, however, should also be considered in selection of equipment.

Methods of Heat Transfer

Heat always flows from a hot to a cold object, in strict compliance with the second law of thermodynamics (Art. 13.2). This direction of heat flow occurs by conduction, convection, or radiation and in any combination of these forms.

Thermal conduction is a process in which heat energy is transferred through matter by the transmission of kinetic energy from molecule to molecule or atom to atom.
Thermal convection is a means of transferring heat in air by natural or forced movements of air or a gas. Natural convection is usually a rotary or circular motion caused by warm air rising and cooler air falling. Convection can be mechanically produced (forced convection), usually by use of a fan or blower.
Thermal radiation transfers energy in wave form from a hot body to a relatively cold body. The transfer occurs independently of any material between the two bodies. Radiation energy is converted energy from one source to a very long wave form of electromagnetic energy. Interception of this long wave by solid matter will convert the radiant energy back to heat.

Thermal Conduction and Conductivity

Thermal conduction is the rate of heat flow across a unit area (usually 1 ft2) from one surface to the opposite surface for a unit temperature difference between the two surfaces and under steady-state conditions. Thus, the heat-flow rate through a plate with unit thickness may be calculated from

The coefficient k depends on the characteristics of the plate. The numerical value of k also depends on the units used for the other variables in Eq. (13.16). When values of k are taken from published tables, units given should be adjusted to agree with the units of the other variables.
In practice, the thickness of building materials often differs from unit thickness.
Consequently, use of a coefficient of conductivity for the entire thickness is advantageous.
This coefficient, called thermal conductance, is derived by dividing the conductivity k by the thickness L, the thickness being the length or path of heat flow.
Thermal Conductance C and Resistanced R. Thermal conductance C is the same as conductivity, except that it is based on a specific thickness, instead of 1 in as for conductivity. Conductance is usually used for assemblies of different materials, such as cast-in-place concrete and concrete block with an airspace between. The flow of heat through such an assembly is very complex and is determined under ideal test conditions. In such tests, conductance is taken as the average heat flow from a unit area of surface (usually 1 ft2) for the total thickness of the assembly.
In the case of 9-in-thick concrete, for example, the conductance, as taken from appropriate tables, would be 0.90 Btu/ (hr)(ft2)(F). (It should be understood, however, that conversion of the conductance C to conductivity k by dividing C by the thickness will produce significant errors.)
Conductance C is calculated from

Thermal resistance R is used in HVAC calculations for determining the rate of heat flow per unit area through a nonhomogeneous material or a group of materials.
Air Films. In addition to its dependence on the thermal conductivity or conductance of a given wall section, roof, or other enclosure, the flow of heat is also dependent on the surface air films on each side of the constructions. These air films are very thin and cling to the exposed surface on each side of the enclosures. Each of the air films possesses thermal conductance, which should always be considered in HVAC calculations.
The indoor air film is denoted by Æ’i and the outdoor film by Æ’o . Values are given in Table 13.3 for these air films and for interior or enclosed air spaces of assemblies.
In this table, the effects of air films along both enclosure surfaces have been taken into account in developing the air-film coefficients. Additional data may be obtained from the ASHRAE Handbook Fundamentals.
Air-to-Air Heat Transfer. In the study of heat flow through an assembly of building materials, it is always assumed that the rate of heat flow is constant and continues without change. In other words, a steady-state condition exists. For such a condition, the rate of heat flow in Btu per hour per unit area can be calculated from

Coefficient of Thermal Transmittance U. The coefficient of thermal transmittance U, also known as the overall coefficient of heat transfer, is the rate of heat flow under steady-state conditions from a unit area from the air on one side to the air on the other side of a material or an assembly when a steady temperature difference exists between the air on both sides.
In calculation of the heat flow through a series of different materials, their individual resistances should be determined and totaled to obtain the total resistance Rt . The coefficient of thermal transmittance is then given by the reciprocal of the total resistance:

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