30 November 1998
b. Control system devices such as dampers and valves are sized on the basis of capacity requirements
and allowable pressure drops and velocity ranges. The selection of the type of valve or damper is based
on factors such as allowable leakage rates and available or practical size ranges.
c. The designer will analyze piping circuit pressure drops and their effects on the system's pressure
drop due to the control valves, based on criteria presented in this manual. The designer will calculate the
required liquid-flow coefficients (Kv (Cv)) for each valve and will show the Kv (Cv) for each valve. The
designer will make sure that pumps are selected to include the pressure drop through the total circuit, and
will then show the pressure against which the valve must close.
d. The designer will size control dampers based on the criteria presented in this manual and will show
the size of each damper.
e. Oversizing of control devices would result in systems in which it would be difficult or impossible to
obtain satisfactory control loop operation, regardless of the quality of the controller and components used.
The designer must not assume that self-tuning controllers will compensate for oversizing of control devices.
The I and D modes compensate for HVAC system load variations and HVAC system equilibrium upsets,
but do not compensate for incorrect valve and damper sizing.
11. SIZING OF THE AIR COMPRESSOR MOTOR.
a. Calculation. The designer will estimate the required air compressor motor size, using equation 2-2,
in order to coordinate with the power circuit serving the air compressor.
Tr = Air compressor running time (percent).
Qc = Control system air consumption in standard milliliters per second (cubic inches per minute (scim)).
Qd = Air compressor delivery in standard milliliters per second (cubic inches per minute (scim)).
b. Running-time criteria. The designer will use the following running-time criteria for sizing air
(1) New air compressors. The running-time design criterion for new air compressors is that the
running time will not exceed 33-1/3 percent. This requires that, initially, the delivery capacity of the air
compressor must be at least three times the estimated air consumption for the whole control system that it
serves. The designer will count the number of each type of air-consuming device shown on the schematics
and apply their characteristic consumption values to arrive at the consumption total. The designer will base
the calculation on the air consumption for each device as shown in table 2-2.
(2) Existing air compressors. Control systems can be added to an existing air compressor until the
running time reaches 50 percent. Exceeding 50 percent run-time risks excessive oil carryover into the air
Table 2-2. Air consumptionof control devices.
Air Consumption (ml/s (scim))