Integration: HVAC fans and smoke control

05/20/2013


Variable frequency drives

The variable frequency drive (VFD) is increasingly becoming a common component of modern smoke control systems. Whether through nondedicated systems where VFDs may be implemented as a means to reduce energy use in HVAC systems or through dedicated systems where VFDs may be used for system balancing, VFDs are required to be listed and conform to recognized standards. For example, all IBC Section 909 requirements that apply for fans and smoke control equipment also apply to VFDs, though they are not specifically mentioned. It is the designer’s responsibility to ensure that VFDs and other sensitive electronic equipment are located in a protected environment and installed so that the expected ambient temperatures do not exceed the limitations of the device. This may be in a temperature-controlled portion of the building, such as a mechanical equipment room or an exterior enclosure, as long as the enclosure can be maintained within the temperature limitations of the equipment. 

As previously mentioned, the IBC requires that fans used for smoke control operate under stable conditions and not exceed their nameplate horsepower rating. Likewise, VFDs are not permitted to be used if their setting could destabilize fan performance or exceed the motor horsepower rating. In addition, the minimum service factor required by the IBC of 1.15 applies. The IBC also requires elements of a smoke management system that rely on volatile memory be supplied with an uninterruptable power supply (UPS) of sufficient duration to span a 15-minute primary power interruption. The designer must take into account potential power surges or interruptions and determine whether a VFD contains nonvolatile memory or if a UPS will be used. This should be clearly identified on the construction documents for each VFD specified for smoke control service. 

VFDs are typically designed with a touchscreen or keypad that can accommodate VFD programming and controls. When the smoke control system is activated either automatically or manually at the firefighter’s control panel, this keypad needs to be overridden and all control/programming functions disabled. In addition, the keypad may have an “off” feature. Although not required by code, it is recommended that the keypad be removed and stored in an approved location. This will ensure that the “off” feature is not misunderstood as a safety override in an alarm mode. VFD manufacturers may require the keypad to be replaced with a blank cover plate in order to maintain the listing of the VFD and protect any connections. It is the designer’s responsibility to determine if protection of the connections is necessary and address the method of protection. 

The IBC requires that fire detection systems be equipped with a control unit complying with UL 864. When the fire alarm control panel signals an alarm to the building management system (BMS), a listing in accordance with UL 864/UUKL is required for the BMS also. UUKL is a listing category under UL 864 for smoke control system equipment. Equipment that receives the UUKL listing has been tested for integrity and long-term reliability.

The UL 864 listing requirement does not necessarily apply to VFDs used for smoke control. VFDs are not required to be UL 864-UUKL listed provided that they are applied as an end device, as they are effectively no different than a motor starter. This implies that the failure of one VFD will only affect the associated fan. However, if the VFD is connected as a controller as part of a control system where its failure will affect smoke control equipment or other operations other than the end device, the VFD may need to be UUKL listed. The designer will need to confirm the application and system architecture when using VFDs as part of a smoke control system. 

Dampers

Figure 3: The combination fire/smoke damper is located in ductwork at a fire-resistance rated barrier. Courtesy: RuskinDampers for smoke control systems are really no different than dampers installed to protect rated assemblies. They are required to be listed and conform to recognized standards. For systems using the IBC as the code, fire dampers are required to meet UL 555. Smoke dampers are required to meet UL 555S. This is consistent with those requirements found in NFPA 92. 

The IBC further regulates the type of smoke damper to be used. They are required to have leakage ratings no less than Class II and elevated temperature ratings of not less than 250 F. This limits the allowable leakage from the damper at the higher temperature expected in the system. 

If combination fire/smoke dampers are used, the smoke damper requirements stay the same, but the temperature rating for the fire damper actuating device is required to be increased to account for elevated temperatures in the system. This is intended to prevent the fire damper from closing when needed for smoke control normal operation. Often fire dampers are selected at lower release temperature for HVAC-only systems. These are typically below the anticipated temperatures in smoke mode. The IBC requires the rating to be approximately 50 F above the normal operating temperature in the duct (in this case under smoke control mode). The operating temperature cannot exceed 350 F. 

One of the things that can be confusing when applying combination fire/smoke dampers in smoke control systems is the elevated temperature requirements for the smoke damper and the elevated temperature requirements for the fire damper. The elevated temperature rating for the smoke damper is 250 F, whereas the fire damper is a maximum 350 F. These are not always the same operating parameters. The elevated temperature of the smoke damper is the temperature at which the damper will continue to maintain minimum leakage per its listing as well as when the damper actuator will still continue to operate. The 350 F fire damper setting is associated with the actuating device. For most combination fire/smoke dampers, the linkage and actuator provide the same function so the two should be coordinated. If the normal operation plus 50 F is less than 250 F, then the minimum elevated temperature requirements for the smoke damper apply; if it is greater than 250 F, then a higher rating is required. 

Smoke dampers found in smoke control systems are generally located at smoke barriers or fire-resistance rated walls. However, as defined they include any damper in the air distribution system, whether it is located in the field or at the unit. HVAC units often use control dampers to mix outside air, return air, and exhaust air. These dampers need to be configured when entering a smoke control mode to allow the system to perform under its expected operation. These dampers also need to be able to prevent the migration of smoke and, per NFPA 92 and IBC, be listed. 



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