Handbook of air conditioning and refrigeration / Shan K

4.2 CHAPTER FOUR
2. Indoor air quality
● Air contaminants
● Outdoor ventilation rate provided
● Air cleanliness for processing
3. Specific design parameters
● Sound level
● Pressure differential between the space and surroundings
The indoor design parameters to be maintained in an air conditioned space are specified in the
design document and become the targets to be achieved during operation. In specifying the indoor
design parameters, the following points need to be considered:
1. Not all the parameters already mentioned need to be specified in every design project. Except
for the indoor air temperature which is always an indoor design parameter in comfort air conditioning,
it is necessary to specify only the parameters which are essential to the particular situation
concerned.
2. Even for process air conditioning systems, the thermal comfort of the workers should also be
considered. Therefore, the indoor design parameters regarding health and thermal comfort for the
occupants form the basis of design criteria.
3. When one is specifying indoor design parameters, economic strategies of initial investment
and energy consumption of the HVAC&R systems must be carefully investigated. Design criteria
should not be set too high or too low. If the design criteria are too high, the result will be an excessively
high investment and energy cost. Design criteria that are too low may produce a poor indoor
air quality, resulting in complaints from the occupants, causing low-quality products, and possibly
leading to expensive system alternations.
4. Each specified indoor design parameter is usually associated with a tolerance indicated
as a � sign, or as an upper or lower limit. Sometimes there is a traditional tolerance understood
by both the designers and the owners of the building. For instance, although the summer
indoor design temperature of a comfort air conditioning system is specified at 75 or 78°F
(23.9 or 25.6°C), in practice a tolerance of �2–3°F (�1.1–1.7°C) is often considered
acceptable.
5. In process air conditioning systems, sometimes a stable indoor environment is more important
than the absolute value of the indoor parameter to be maintained. For example, it may not be necessary
to maintain 68°F (20°C) for all areas in precision machinery manufacturing. More often, a
72°F (22.2°C) or even a still higher indoor temperature with appropriate tolerance will be more
suitable and economical.
4.2 HEAT EXCHANGE BETWEEN HUMAN BODY AND
INDOOR ENVIRONMENT
Two-Node Model of Thermal Interaction
In 1971, Gagge et al. recommended a two-node model of human thermal interaction. In this model,
the human body is composed of two compartments: an inner body core, including skeleton, muscle
and internal organs; and an outer shell of skin surface. The temperatures of the body core and the
surface skin are each assumed to be uniform and independent. Metabolic heat production, external
mechanical work, and respiratory losses occur only in the body core. Heat exchange between the
body core and the skin surface depends on heat conduction from direct contact and the peripheral
blood flow of the thermoregulatory mechanism of the human body.