HVAC and Fire Safety for Elevator Systems
Elevator industry resources estimate that there are about 900,000 installed elevator systems in the United States. Hydraulic elevators constitute nearly 70% of the vertical transportation (VT) market and 30% are of the traction type. Hydraulic elevators typically are installed in low- and mid-rise buildings as high as four stories.
Elevator industry resources estimate that there are about 900,000 installed elevator systems in the United States. Hydraulic elevators constitute nearly 70% of the vertical transportation (VT) market and 30% are of the traction type. Hydraulic elevators typically are installed in low- and mid-rise buildings as high as four stories. Traction elevators are installed in buildings of all heights but have the advantage of higher speed for taller buildings.
In addition, engineers must ensure that elevators' hoistways conform to strict NFPA requirements. Smoke control in elevator hoistways, elevator lobbies and stairways must be controlled by a UL 864 listed alarm system meeting the requirements of NFPA 72. Elevator cartop electrical components should be enclosed in NEMA 4 enclosures to resist water that may drip on top of the car while the fire is being extinguished.
A new type of elevator that has been aggressively gaining momentum in the VT market is the machine room-less (MRL) elevator. MRL elevators can be used efficiently in buildings as high as four stories and have the advantage of not requiring dedicated non-leasable area for equipment. MRL elevators still require a small machine room to house the controller and a man-size access door to the hoistway for access to the elevator machine, which is mounted inside the hoistway as part of the ropes-and-sheave system. While eliminating the bulk of the machine room, MRL elevators still share some HVAC design requirements found in hydraulic and traction elevator hoistways and cars.
Hydraulic and traction elevators require machine rooms adjacent to the elevator hoistway and therefore take up leasable area in buildings, while requiring additional support services that include dedicated HVAC. The mechanical engineer's design that relates to the elevator system and ventilation requirements for the machine room and elevator hoistways are a key to the safe and efficient operation of a building system.
Traction elevator machine room
This room normally houses the elevators, machines, controllers, governors and related electrical components. Room size is approximately 12 ft. x 12 ft. for one elevator system including maintenance space requirements, or 20 ft. x 12 ft. for two systems. Traction elevator machine rooms, while separate from the elevator hoistway, docommunicate by multiple openings for ropes or cables serving the cars that introduce air flows into the machine room.
The major heat generating equipment in the machine room is the elevator machine motor that raises and lowers the elevator car through several steel ropes, and the controller. The controller must operate within temperature parameters and therefore is fan vented. Simple controller ventilations systems draw machine room air into the controller enclosure, pass it over the electronic equipment and discharge it back into the machine room, where the machine room AC system is able to handle the cooling load. Machine room air that serves the controller in addition to the remainder of the space has to be clean with filtration covers on the machine ventilation discharge opening where carbon particles also are discharged.
Hydraulic elevator machine room
The hydraulic elevator machine room is usually 25% to 50% smaller than the traction elevator machine room. There is no air transfer relationship with the hoistway as with traction elevators because only a 2-in. wide steel pipe connects the hydraulic machine and the hoistway piston. The penetration openings to the hoistway are caulked with a UL fire-resistant rated sealant to maintain the hoistway fire rating and ensure a smoke tight seal as well. The 2-in. wide steel pipe carries 400 psig hydraulic oil that is used to raise or lower the car cylinder.
The hydraulic unit and elevator controller generate heat, therefore air conditioning is needed for the machine room in order to keep the hydraulic oil temperature at normal operating temperatures. AC system capacities are similar to traction elevator machine rooms.
Machine room design
When designing an elevator machine room HVAC system, a cooling load calculation is required to determine the BTU/hr that the AC system requires. The elevator machine HVAC system, due to its exposure to the equipment and a need to have it operate off of emergency generators, leads the designer to an independent system separate from the building HVAC system. A typical 3,000-lb. capacity elevator usually requires a 1.5-ton to 2-ton AC system depending on the machine room's location in the building and the climate for the local. ASME A17.1, “The Safety Code for Elevators and Escalators,” requires machine room temperature to be as determined by the elevator manufacturer. Most building elevator mechanics and manufacturers maintain elevator machine rooms between 60°F and 80°F with 35% to 60% relative humidity. The heating system for the machine room also is a built-in unit.
AC systems for machine rooms
The most common and practical AC systems for machine rooms are the ductless split type with automatic heating/cooling heating switchover (see Figure 1). This system's lifetime is approximately 10 years and costs $8,000 to $13,000 (installed). Capacities of the ductless split AC range from 6,000 BTU/hr. to 48,000 BTU/hr.
In a survey of machine room AC systems, one manufacturer had a 60,000-BTU/hr. cooling capacity for one- or two-unit rooms. For semi-standby AC systems, it is acceptable to provide two identical split type AC systems, each handling 50% of the load. For hospitals, the standby system is designed for 100% capacity and must be interchangeable in operation bimonthly to maintain reliability through use. In colder climates, machine room systems should be specified with low outdoor ambient temperature down to 0°F because the machine room may still require cooling during outdoor winter days.
Condensation from AC systems in machine rooms is handled by an insulated condensation waste drips into an indoor open, vented sanitary waste system. No floor drains are allowed in machine rooms and curbing from the remainder of the floor is not uncommon. In warmer climates, a dry well into the ground may be used for condensate waste discharge. A dual-condensate waste pump is recommended because machine rooms are unoccupied 90% of the time.
While some buildings are provided with chilled and hot water systems for HVAC applications, serving the elevator machine room with one of or both of these systems is not recommended because any water leakage could be disastrous to the elevator system. Additionally, the building could be in the heating mode while the elevator machine room may require cooling.
In renovation design such as an elevator system modernization or a replacement project, most older machine rooms and controllers were ventilated with outside air using a motorized damper with an air intake louve, interlocked with a wall- or roof-mounted exhaust fan. When replacing the elevator system, it is best to remove this system and install a new split AC unit because the existing ventilation system may not meet the new cooling load requirements. The microprocessor components of the controller are more sensitive to heat and humidity than older relay controllers.
The renovated machine room should not have any type of air transfer openings between it and the hoistway. Any existing floor openings should be closed off, covered with steel plates that are flush with the floor and caulked airtight. The only unavoidable air transfer is at the holes in the floor where car ropes travel between elevator car and elevator machine. The piston action, due to elevator car movement inside the hoistway, will cause air transfer between the machine room and the hoistway. The loss of air between the hoistway and machine room should be adjusted for in the cooling load calculation for the machine room AC.
The capacity of the emergency generator during a power outage in case of fire or other emergency must also handle the load of at least one elevator car that transits the entire building. The machine room AC system and elevator machine and controller electrical loads must be included in emergency electric loads. Where the building code designs allow for the installation of shaft pressurization systems for fire emergencies, the load of these systems must be added into emergency generator capacity.
Elevator hoistway ventilation
AC systems for air in elevator cars are important to the comfort of occupants that use building elevators. Most car systems consist of an air-cooled car top unit that will operate with 100% return air. The car AC is most commonly found in high-rise buildings where high occupancy levels of larger cars combined with increased travel times require additional ventilation. A car ventilation system operates through air intake slots at the car floor level where air is drawn from the hoistway and discharged at the top of the car through a 12-in. diameter exhaust fan that discharges air back into the hoistway. The ventilation fan capacity is sized to the car volume or 3.5 times the car floor area, whichever is greater. When the car AC system is operational, the exhaust fan is automatically turned off. Installation of packaged AC systems on top of elevator cars should follow manufacturer recommendations for mounting and clearance from equipment including the escape hatch and its opening.
Air conditioned elevator machine rooms located in an unconditioned basement level should be provided with vapor retardant wall covers. In some cases, this may not be adequate as water vapor condenses on cold metals inside machine room including electrical boxes and conduit. For such applications, a dehumidification system should be considered to eliminate or reduce condensation.
In cold climate regions, where ambient outdoor temperature may reach 0°F, hoistways for parking garages or other buildings and locations where part of the hoistway walls are exposed to cold winds and low ambient temperatures may reach temperatures below freezing. Such temperatures contribute to exposed metal corrosion including the car exterior, hoistway entrance doors, sills, toe guards and headers. Because the mechanical engineer designs both the sprinkler system and the hoistway heating, the HVAC system should keep the sprinkler from freezing. A heating system often is required to keep hoistway equipment at temperatures in the range of 55°F to 60°F in colder climates while also helping to inhibit condensation and corrosion. The elevator code allows UL-rated electric heaters to be installed at different heights in the hoistway, one at the pit, one at the middle of hoistway and one at the highest floor level. Each heater has a built-in thermostat.
Present codes do not specifically recommend any type of ventilation system for the hoistway under normal operations except a vent at the top of the hoistway that automatically opens to relieve the piston pressure of elevator car movement and possibly vent some of the smoke that might collect in the elevator shaft in the event of a fire.
Hoistway pressurization in fire emergencies
Several states, most notably Oregon and Washington, have had hoistway pressurization provisions for fire emergencies as a method of keeping smoke from moving into shafts and then onto other floors. The International Building Code in the 2006 edition introduced provisions in Section 707.14.2 for the use of elevator hoistway pressurization in lieu of elevator lobbies or opening.
ASME, in a joint task group with the National Fire Protection Assn. and the International Code Congress, is examining firefighter use of elevators in a fire emergency and the use of elevators by building occupants in fire and other emergencies. The task groups also are developing proposals for changing provisions in elevator, building and fire codes (see Figure 2).
Meanwhile, recent studies by B44 Canadian elevator code elevator and fire protection systems experts, in the ANSI A17.1 elevator committee group have been suggesting several hoistway pressurization concepts.
The most popular concept has been the use of supply fans to pressurize the hoistway such that the elevator system may be used by trained firemen to safely transport people during fires. This pressurization technique will involve the use of variable speed fans controlled by variable speed drives and static pressure sensors. This method of hoistway pressurization will have to include machine-room positive pressurization because machine room air is intermixed with hoistway air through the ropes that travel openings in the floor. The positive pressurization system would only operate during a fire. The duration of operation is determined by the firefighting personnel. The elevator lobby, stairways and corridors each must have its ownindependent pressurization control system in order to achieve reasonable control of smoke and allow safe passage to occupants.
Architects and contractors who are involved in hoistway design and construction must make the elevator hoistway an airtight structure in compliance with Construction Standards Institute (CSI) elevator shaft construction standards so that during fire a positive pressure is automatically kept at 0.05 in. W.G.
The pressurization sensing controls should take into consideration the piston effect due to the car movement inside the elevator. In addition, the building's HVAC system should not seriously interfere with this positive pressurization system for the elevator hoistway and elevator lobbies.
The report, “Building Standards,” by V.R. Bush, P.E., published information on the MGM Grand Hotel fire in Las Vegas in February 1981. This report said that smoke and gases, which raced up the hotel elevator hoistway, were trapped at the top of the hoistway due to failure of an automatic vent damper to open. With the pressure buildup, smoke diverted into the 26th floor and then out into the corridors.
The features that contributed to the spread of fire and smoke through the building were:
No elevator hoistway pressurization.
No elevators lobby areas pressurization.
No stairwell pressurization.
Inadequate sealing between the elevator hoistway doors and the door framing at the lobbies, which allowed smoke from the hoistway to enter and escape into the hotel corridors and stairwells.
Use of elevators during fires
Smoke control in elevator hoistway, elevator lobbies and stairways must be controlled by a UL 864 listed alarm system meeting the requirements of NFPA 72. Elevator cartop electrical components should be enclosed in a NEMA 4 enclosure to resist water that may drip on top of the car, during the fire extinguishing.
Evacuation of passengers from elevators shall comply with ASME A17.4 “Guide for Emergency Evacuation of Passengers from Elevators,” 1999.
Elevator hoistways should be mechanically pressurized with outside air when activated by any manual or automatic alarm-initiating device or fire sprinkler water flow, to maintain a positive pressure of 0.05-in. W.G. An elevator hoistway pressurization system should not have fire or smoke dampers.
A pressurization fan must be provided with a smoke detector wired to the fire alarm system. Pressurization ductwork system connecting the pressurization air handling unit to the hoistway penetration shall be enclosed in a 2-hour fire resistive enclosure. Turning off the hoistway pressurization system should only be controlled by trained firemen. Smoke venting of a hoistway to the outdoors is prohibited. The elevator hoistway's pressurization systems should be independent from other systems. Because this article only deals with elevator issues, no further discussion is extended to cover the elevator lobby and the stairwells pressurization.
Elevator systems designs often are determined by the architects who request a layout and specifications from the elevator manufacturer representatives. In this instance, the architect will eliminate the mechanical engineer's participation in the elevator design, and the important considerations for proper HVAC design of machine rooms and shaftways also will be left out.
It is important that the elevator system designer coordinates with the mechanical engineer, as well as with the fire protection and electrical engineers to ensure the integration of all environmental and safety requirements for the elevator system. When all of the design elements are properly considered, the operation of one of the most important public elements of the building will work efficiently and reliably.