Application Details; Multi-Zone Buildings With Varying Loads; Cooling - Carrier 42BJ ULTRA Selection Manual

4 - APPLICATION DETAILS

4.1 - Multi-zone buildings with varying loads

When drawing up the thermal load schedule, the following
thermal gains and losses in the peripheral areas of the building
must be taken into account:
• Losses through outside walls and glazing,
• Solar gains, and heat gains from lighting, people and equip-
ment (computers, copiers etc.).
The concept of the 42BJ ULTRA is based on controlling two
separate fluids to compensate for variations in the cooling and
heating loads imposed in the peripheral areas of the building.
These two fluids are chilled air and heated air which are both
generated within the ULTRA and delivered to strategically
positioned diffusers.
The sensible cooling load which has to be taken into account in
order to counter solar gains, and heat gains from lighting and
the building's occupants, varies with the orientation and occu-
pancy of the building (Fig. 1). Transmission gains, on the other
hand, depend only upon the temperature difference between the
interior of the building and the outdoors (Fig. 2).
The outdoor air temperature, time of day, exposure and season
together enable the required cooling and heating capacities to
be calculated. Thus it is known that in the northern hemisphere
the maximum thermal gains for rooms with easterly or westerly
exposures will occur in July or August. Tpipe with southerly
exposures will have their maximum thermal gains between
September and November, depending upon the amount of
external glazing and the type of building construction. The
combined effects of sun, lights and people can change a heating
load into a cooling load, even in winter with a very low outdoor
temperature.
Lights, people and solar gains
Fig. 1. Typical building loads
EAST WEST
SOUTH
Lights
Lights and
people
July
These variable cooling and heating loads (Fig. 3) show the need
for a system which uses two fluids in such a way that it can
maintain acceptable conditions in all areas of the building at all
times.

4.2 - Cooling

Supply air from the ULTRA cools and mixes with outdoor air
from a central air handler, and is then ducted to diffusers (typi-
cally 35BD/35SR) located in the false ceiling of the room.
An appropriate volume of chilled, dehumidified air is introdu-
ced into the room to compensate for the total heat gains from
the sun, people, lights, equipment and cooling losses.
The ULTRA automatically responds to variations in the inter-
nal and external loads by varying its flow of cooled air so as to
stabilise the temperature in the air conditioned space at the
desired level.
Fig. 2. Typical transmission load
Fig. 3. Typical air conditioning load
Outdoor air temperature (°C)
Outdoor air temperature (°C)
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