Series 100 Single Package Units, Engineering Guide, 100.50-EG7

Application Data (continued)
ECONOMIZER
The economizer section is used for ventilation of the
conditioned space to maintain indoor air quality, and
also to reduce energy consumption by using outdoor
air cooling in lieu of mechanical cooling. If outdoor air is
appropriate for cooling, but not suffi cient for the cooling
demand, mechanical cooling will stage on as necessary
until the cooling load is met.
Dual (comparative or differential) enthalpy operation is
the most accurate and effi cient means of economizer
operation. The IPU controller monitors the return and
outside air energy content, and selects the lower of the
two for operation.
VAV SUPPLY AIR PRESSURE CONTROL
Traditional packaged single package unit systems
use inlet guide vanes (IGVs) for duct static pressure
control. These control supply duct pressure by modulating
dampers (introducing losses and ineffi ciencies)
on the inlet of the fan, open and closed. JOHNSON
CONTROLS variable frequency drives (VFDs) offer
superior fan speed control and quieter, energy effi cient
operation.
FIG 1: TRADITIONAL OVERHEAD VAV AIR
DELIVERY SYSTEM
For VAV applications, the JOHNSON CONTROLS Series
100 unit uses a VFD to modulate fan speed and
maintain a constant duct static pressure. VFDs offer
superior control over the operation of the unit at part
load, and offer the additional benefi ts of quieter and
more effi cient operation when compared to IGV.
FLEXSYS
The traditional approach to HVAC design in commercial
buildings has been to supply conditioned air through
14
FORM 100.50-EG7 (1110)
extensive overhead duct networks to an array of diffusers
spaced evenly in the ceiling. In Figure 1, the conditioned
air is both supplied and returned at ceiling level. Ceiling
plenums must be designed large enough to accommodate
the supply ducts that run through them. Return air
is typically confi gured as ceiling plenum return without
any ductwork. This type of air distribution, known as the
“well-mixed” type, is the most common system in use.
This conventional HVAC system is designed to promote
complete mixing of supply air with room air, thereby
maintaining the entire volume of all air in the space (from
fl oor to ceiling) at the desired space setpoint temperature.
In addition, to meet IAQ requirements, an adequate
supply of fresh outside air must be introduced to this mix.
A key disadvantage to this control strategy is that it has
no provisions to accommodate different temperature
preferences among the building occupants or to provide
preferential ventilation in the occupied zone.
FIG. 2: JOHNSON CONTROLS FLEXSYS UNDER-
FLOOR AIR DELIVERY SYSTEM
With the JOHNSON CONTROLS FlexSys Underfl oor
Air System, conditioned air from the air handling unit
(AHU) is ducted to the underfl oor plenum. As shown in
Figure 2, this conditioned air fl ows freely throughout
the plenum to individual supply discharge outlets. Unlike
the larger single supply duct outlets typical of overhead
systems, underfl oor systems are confi gured to
have a large number of smaller supply outlets, in close
proximity to the building occupants. These adjustable
outlets provide an opportunity for nearby occupants to
have some amount of control over thermal comfort
conditions in their local environment. Air is returned from
the room at ceiling level (unducted plenum return is
shown). The resulting overall fl oor-to-ceiling air fl ow
pattern takes advantage of the natural buoyancy produced
by heat sources in the space and more effi -
ciently removes heat loads and contaminants from the
space, particularly for cooling applications. In fact, some
of the most important advantages of underfl oor systems
JOHNSON CONTROLS