Handbook of air conditioning and refrigeration / Shan K

System Capacity
Indoor Air Quality
AC SYSTEMS: SYSTEM CLASSIFICATION, SELECTION 26.5
A four-pipe fan-coil system is the most widely used air conditioning system for guest rooms
in luxury hotels. This is because a four-pipe fan-coil system can provide individual temperature
and fan speed controls as well as a positive supply of adequate outdoor ventilation air. The fan
coil can be turned off conveniently when the room is not occupied. It is isolated acoustically
from adjacent rooms. The most annoying space maintenance tasks, such as changing the filters
and periodic inspection and maintenance of the fan coil, can be done when the guest room is
unoccupied.
Specific design criteria are always related to applications. Specific design criteria usually dictate
the type of air conditioning system that should be selected. For instance, a high-precision constanttemperature
room maintained at a temperature of 70 � 0.2°F (21 � 0.1°C) within a limited working
space needs a constant-volume central system with electric terminal reheat. Electric reheat can
be modulated more precisely than any other type of heating system. A central system can maintain
a more uniform discharge air temperature at the AHU than DX coil packaged units. Usually, such a
high-precision constant-temperature room is a clean space. A constant-volume system is always
preferable to a VAV system for such rooms.
System capacity is closely related to the selection of an air conditioning system. For a single-story
small retail shop, a constant-volume packaged system is often chosen. If the conditioned space is a
large indoor stadium with a seating capacity of 70,000 spectators, a single-zone VAV central system
with many AHUs using peripheral nozzles at different levels to supply conditioned air is often
selected.
Indoor air quality (IAQ) depends mainly on the minimum ventilation control; the removal of bacteria,
particulates, irritating vapors, and toxic gases; preventing dampness; and proper maintenance of
the HVAC&R system. Among various minimum ventilation controls, the dedicated ventilation system
with CO 2-based demand-controlled ventilation guarantees the minimum outdoor ventilation
rate specified by ASHRAE Standard 62-1999 and is energy-efficient.
The dedicated ventilation system, the mixed plenum pressure control, and the outdoor air injection
fan are good minimum ventilation control systems with satisfactory performance. All guarantee
the specified constant-volume flow of minimum ventilation rate at the outdoor air intake in the
AHU or PU.
The supply and return fan tracking systems, and the direct measurement of outdoor air intake in
the AHU and PU are considered as poor minimum ventilation control systems. As the volume flow
rate of the outdoor ventilation air is usually less than one-fourth that of the total supply air, it is
difficult to measure it correctly by measuring the volume flow rate of supply and return air. This
may result in an insufficient outdoor ventilation intake, especially during part-load operation in
VAV systems.
Air economizer cycle and purging operation prior to the occupied period on weekday mornings
both improve IAQ because of the introduction of all outdoor air to the conditioned space.
As discussed in Secs. 15.16 and 24.5, to remove particulates, bacteria, viruses, irritating vapors,
and toxic gases from the conditioned space, air filtration and gaseous adsorption and chemisorption
in the AHUs or PU can be rated into the following four grades:
● Low-efficiency air filters are used to remove dusts of size 3 to 10 mm, such as, spores, pollens,
and textile fibers. Panel filters made of wire mesh and synthetic and glass fibers are often used as
the filtration medium.
● Medium-efficiency filters are used to remove dusts of size 1 to 3 mm, such as, bacteria, automotive-emissions,
and fine dust for the protection of coils and air distribution systems and to prevent