Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

Page: 19 / 512

ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
9
Table 1. Functions of Central HVAC Control Loops.
HEATING
GENERAL
Building heat loss occurs mainly through transmission,
infiltration/exfiltration, and ventilation (Fig. 3).
Fig. 3. Heat Loss from a Building.
The heating capacity required for a building depends on the
design temperature, the quantity of outdoor air used, and the
physical activity of the occupants. Prevailing winds affect the
rate of heat loss and the degree of infiltration. The heating
system must be sized to heat the building at the coldest outdoor
temperature the building is likely to experience (outdoor design
temperature).
Transmission is the process by which energy enters or leaves
a space through exterior surfaces. The rate of energy
transmission is calculated by subtracting the outdoor
temperature from the indoor temperature and multiplying the
result by the heat transfer coefficient of the surface materials.
The rate of transmission varies with the thickness and
construction of the exterior surfaces but is calculated the same
way for all exterior surfaces:
Energy Transmission per
Unit Area and Unit Time = (T
IN
- T
OUT
) x HTC
Where:
T
IN
= indoor temperature
T
OUT
= outdoor temperature
HTC = heat transfer coefficient
HTC
=
joule
Unit Time x Unit Area x Unit Temperature
ROOF
-7
°
C
TRANSMISSION
VENTILATION DUCT
EXFILTRATION
DOOR
WINDOW
PREVAILING
WINDS
INFILTRATION
20
°
C
C3971
Control
Loop Classification Description
Ventilation Basic Coordinates operation of the outdoor, return, and exhaust air dampers to maintain
the proper amount of ventilation air. Low-temperature protection is often required.
Better Measures and controls the volume of outdoor air to provide the proper mix of
outdoor and return air under varying indoor conditions (essential in variable air
volume systems). Low-temperature protection may be required.
Cooling Chiller control Maintains chiller discharge water at preset temperature or resets temperature
according to demand.
Cooling tower
control Controls cooling tower fans to provide the coolest water practical under existing
wet bulb temperature conditions.
Water coil control Adjusts chilled water flow to maintain temperature.
Direct expansion
(DX) system control Cycles compressor or DX coil solenoid valves to maintain temperature. If
compressor is unloading type, cylinders are unloaded as required to maintain
temperature.
Fan Basic Turns on supply and return fans during occupied periods and cycles them as
required during unoccupied periods.
Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces system
operating cost and improves performance (essential for variable air volume systems).
Heating Coil control Adjusts water or steam flow or electric heat to maintain temperature.
Boiler control Operates burner to maintain proper discharge steam pressure or water temperature.
For maximum efficiency in a hot water system, water temperature should be reset as
a function of demand or outdoor temperature.