Handbook of air conditioning and refrigeration / Shan K

A VAV packaged system is shut off during nighttime unoccupied hours in summer and off seasons.
In locations where there is a cold winter, it operates to provide warm air and maintain a night
setback indoor space temperature during unoccupied hours.
Air-Cooled, Water-Cooled, and Evaporatively Cooled Condensers
In rooftop packaged units using reciprocating compressors, the energy efficiency ratio (EER) for
air-cooled, water-cooled, and evaporatively cooled condensers at an outdoor dry-bulb temperature
of 95°F (35°C) and a wet-bulb temperature of 75°F (23.9°C) is as follows:
Air-cooled condenser 9–12
Water-cooled condenser 11–14
Evaporative-cooled condenser 14–16
The EER for VAV packaged units using scroll compressors is higher than that for those using
reciprocating compressors. Air-cooled and evaporatively cooled condensers often have a higher installation
cost than water-cooled condensers and cooling towers.
Although water-cooled and evaporatively cooled condensers are considerably more energy-efficient
at an outdoor condition of 95°F (35°C) dry-bulb and 75°F (23.9°C) wet-bulb temperatures
than air-cooled condensers, in many locations the annual average dry-bulb temperature is rather
low. Shaffer (1987) used a manufacturer’s computer program to compare the annual energy use of
air-cooled and water-cooled reciprocating chillers for multistory buildings using VAV reheat systems
in Boston and San Diego. The annual energy use of a water-cooled condenser and cooling
tower is about 10 percent higher than that of an air-cooled condenser, mainly due to the energy use
of condenser pumps. Comparing the energy use and costs of air-cooled, water-cooled, and evaporatively
cooled condensers is recommended in order to make an energy-efficient and cost-effective selection.
Controls and Sound Problems for Rooftop Packaged Units
AC SYSTEMS: PACKAGED AND DESICCANT-BASED SYSTEMS 29.17
Microprocessor-based specific controls, safety controls, and diagnostics for a 1997 manufactured
rooftop packaged unit with a cooling capacity between 20 to 130 tons (70 to 457 kW) are listed in
in Sec. 16.5. Noise considerations for rooftop top packaged units are discussed in Sec. 19.9.
Case Study: VAV Reheat Packaged System for Precision Manufacturing
Desmone and Frank (1992) reported an addition of 10,000 ft 2 (930 m 2 ) for sophisticated precision
manufacturing for Oberg Industries, a manufacturer of high-quality carbide and steel stamping dies
in Freeport, Pennsylvania. The required HVAC&R criteria are as follows:
● Temperature is maintained at a constant level of 68 � 1°F (20 � 0.56°C) not only horizontally
but also vertically from the floor to 13 ft (3.96 m) high.
● Humidity should be maintained at 50 percent.
● Airflow noise had to be limited and drafts and turbulence should be tightly constrained.
The selected HVAC&R system is a VAV reheat packaged system with two rooftop packaged units
that feed cold air year-round into a medium-pressure duct loop. Machine oil duct exhaust, grinding
duct exhaust, conditioned air return, medium-pressure supply duct, and the lowest VAV boxes and
reheating hot water piping are arranged in layers to facilitate servicing.
There are 19 pneumatically controlled VAV boxes located throughout the extended plant with a
two-stage modulation in sequence. The modulation of the supply air volume flow by the damper in