Ventilating the Machine Room
“That is part of the reason IIAR is instituting a certificate program,” said Dave Schaefer, chief engineer at Bassett Mechanical in Kaukauna, Wisconsin. “Obtaining the IIAR-2 certificate will confirm that an engineer has studied the design and safety standards and have shown that the individual understands the requirements.” He explained recommended practices for proper ventilation are just one of many important safety design elements found in IIAR-2.
The first step in properly ventilating a machinery room begins with the design of the ventilation system. In the event of a leak, it is vital that ammonia vapor is dispersed as high into the air as possible so that when it returns to ground level it is no longer hazardous to people. The IIAR-2 standard requires a minimum of 2,500 feet-per-minute discharge velocity. The discharge needs to be 20 feet from a lot line or building opening. “You don’t want to use mushroom style or sidewall fans,” Schaefer said.
“You want to blast the ammonia vapor straight up into the air so it dissipates quickly into the atmosphere. You don’t want it blowing into another part of the building. You want to get the discharge up and away from the facility and dilute it.”
Fans must be sized appropriately in relation to the volume of the room so that they can provide at least 30 air changes per hour. “So in other words, you have to replace the air in the room every two minutes,” Schaefer said.
A proper design brings fresh air into the room at one end and sweeps that fresh air across the room. It is most efficient to place exhaust fans on the roof above an area where an ammonia leak is most likely to occur, such as near re-circulators or other critical equipment in the machinery room. The design, which could also feature more than one fan, can also be used for temperature-control ventilation to cool the room and maintain the required machinery room temperature of 104 degrees or less.
Fan blades must be non-sparking so as not to ignite a fire. When the emergency exhaust system turns on automatically, the system should be controlled so that it must be turned off manually. The manual off control is required because the fan could be damaged if it is allowed to repeatedly cycle on and off and to assure the situation is under control before stopping ventilation.
If the ammonia vapor concentration were to reach a level of 40,000 ppm or more, turn the equipment in the machinery room off, including all compressors and pumps. The ventilation system needs to stay in operation in the event of an emergency, he said. The emergency ventilation system should be powered independently with a separate circuit from outside the machinery room. The ventilation system must have an Auto/On, be clearly identified and have a tamper resistant start switch outside the primary door. Further, intake air dampers need to fail in the open position in the event of loss of power.
Facilities located in cold climates should consider heat trace and insulating the water lines in the machinery room. With the requirements for leak detection having been lowered from 1,000 parts-per-million to 150 ppm, water freezing becomes an issue because the ventilation system now will start operating at the lower 150 ppm level.
Management of the ventilation system requires monitoring the emergency ventilation fans on a 24/7 basis to ensure proper operation. “You have to ensure there is air flow across the fan with a pressure differential sensor or make sure the amp draw is sufficient,” Schaefer said. “If it’s not pushing the air, the fan motor won’t draw as many amps. You have to make sure it’s moving the right amount of air.”
In the event of an ammonia leak, sweeping the room with fresh air requires the proper placement of the exhaust fan and intake air. The intake that provides fresh air could be a louver, a louver with a damper or a fan that supplies the air. “The designer needs to make sure the air doesn’t short-circuit because you want to provide fresh air to every part of that room to keep the ammonia concentration to an acceptable level,” Schaefer said.
One leak-related issue to be aware of is the presence of liquid ammonia on the floor. Spraying water onto the liquid ammonia will flash that liquid into the air and create a potentially high concentration of ammonia vapor in the room. The wiser option is to utilize absorbents or allow the liquid to boil off naturally.
Under IIAR-2 design and safety standards issued in 2014, the engine room must have continuous ventilation when it is occupied. Requirements now state that one-half cubic foot per minute, per square foot or 20 cfm per occupant is needed, or whichever is greater.
“We see a lot of machinery rooms that are not designed to IIAR-2 design and safety standards, so education in this area is very important,” Schaefer said. “Ammonia ventilation systems are one of the most important safety systems we have, so they need to be designed and installed properly.”
