Handbook of air conditioning and refrigeration / Shan K

Process Air Conditioning Systems
4.9 HUMIDITY
For process air conditioning systems, indoor design temperature is usually based on previous experiences.
For precision manufacturing projects, a basic temperature plus a tolerance, such as 72 �
2°F (22.2 � 1.1°C) for a precision machinery assembling workshop, is often specified. First, the
tolerance should be neither too tight nor too loose. Second, either the temperature fluctuation at
various times within the working period or the temperature variation within the working space,
or both, should be included in this tolerance.
In unidirectional-flow clean rooms, the velocity of the airstream in the working area is often
specified as 90 � 20 fpm (0.45 � 0.1 m/s) to prevent contamination of the products.
Comfort Air Conditioning Systems
INDOOR AND OUTDOOR DESIGN CONDITIONS 4.23
According to ANSI/ASHRAE Standard 55-1992, for the zone occupied by people engaged in light,
primarily sedentary activity (� 1.2 met), the relative humidity should conform with the limits of
ASHRAE winter and summer comfort zones, as shown in Fig. 4.6. These limits are intended to
maintain acceptable thermal conditions for the occupants’ comfort.
ASHRAE/IESNA Standard 90.1-1999 mandates that where a zone is served by system(s) with
both humidification or dehumidification capacity, means shall be provided to prevent simultaneous
operation of humidification and dehumidification equipment. Standard 90.1-1999 also specifies that
where humidistatic controls are provided, such controls shall be capable of preventing reheating,
mixing of hot and cold air streams, and simultaneous heating and cooling. Refer to Section 29.12
for more details.
The following results should be considered during the design and evaluation of the performance
of comfort air conditioning systems:
● Maintaining an indoor space relative humidity � r between 20 and 30 percent in winter prevents or
reduces the condensation at the inner side of the window glass.
● In high-occupancy applications, it may be economical and is still comfortable to specify the summer
indoor relative humidity at 55 to 60 percent if the indoor temperature is within the summer
comfort zone.
● The indoor relative humidity at part load may be considerably higher than at full load in some air
conditioning systems in summer.
● When the indoor relative humidity is below 25 percent, the incidence of respiratory infections increases
significantly. If, simultaneously, indoor temperatures are low, such as below 70°F (21°C),
the induced static electricity in carpeted rooms may cause uncomfortable shocks to occupants
contacting metal furniture or decorations.
● Because the increase in the outdoor ventilation rates in ASHRAE Standard 62-1999 for the air
conditioning system serves the building located in areas where the humidity ratio of outdoor air is
very low during winter, the system has greater difficulty maintaining an indoor relative humidty
of 20 to 30 percent without winter humidification.
● If a humidifier is installed, its humidifying capacity should not exceed the actual humidifying
requirements so that wet surfaces do not occur inside the air-handling unit, packaged unit, and
supply ducts. Wet surface and dirt cause the growth of microorganisms and poor indoor air quality.
Therefore, for comfort air conditioning systems, the recommended indoor relative humidity