PIM™ - PRESSURE INDEPENDENCE MODULE is designed for ease of installation with actuator,
controls and transducer factory assembled in a compact package. |
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System Design
Discharge Damper
The static pressure in the duct to the Therma-Fuser is maintained by dampering air flow between fan and low pressure duct. Typical
applications include (1) uncontrolled fan or (2) a fan that has controls that do not lower cfm to the minimum leakage of the Therma-Fuser.
Discharge dampers are not recommended for DX equipment without unloading.
Zone Damper
Static pressure control is achieved by dampering air flow to low pressure ductwork. A typical example is a large system in which supply main
ducts operate at pressures higher than desired for quiet Therma-Fuser operation, about .25" wg.
In variable air volume systems, fan capacity control should be considered for energy reduction. Also, some fans may have a pressure
capability that could damage ductwork if totally closed off. For more on fan control, see option F of Options of Supply Air Static Pressure
Control To Provide Pressure Independence At Therma-Fusers.
Bypass Damper
Static pressure control is achieved by bypassing air from the supply duct to the return. Typical applications include (1) uncontrolled fan
capacity or (2) a fan that has controls that do not reduce capacity to the minimum leakage of the Therma-Fusers.
Bypass dampers should be sized to handle an air volume equal to the total turndown of the Therma-Fusers.
Example 1: 12 Therma-Fuser model TB-10". Design air flow 450 cfm each. Turndown to tight shut off. Size the bypass damper for 5400 cfm.
Example 2: 5 Therma-Fuser model TF-HC-8". Design air flow 275 cfm each. Minimum flow stops set at 20% of design air flow. Size the
bypass damper for 1100 cfm.
Duct Design
If the Therma-Fusers are to deliver nominal capacity at .15" wg in the Therma-Fuser neck and if a maximum of .25" wg is to be held for quiet
operation, the duct between points X and Y should be sized for a maximum pressure drop of .1" wg. In an existing system, if the pressure drop
between X and Y exceeds .1" wg consider additional dampering, relief or a parallel duct. If Therma-Fuser design is below nominal, say .05"
wg in the Therma-Fuser neck the duct between X and Y can be sized for a maximum pressure drop of .2" wg.
