Machinery Room ventilation is the first and most important design of an ammonia machinery room. Sadly, its geometry is often determined by an architect or general contractor that only thinks cost.
Machinery Room location is paramount in terms of accomplishing good ventilation. In years past some of the dairies put Machinery Rooms in basements. It was almost a tradition in that the Machinery Room would be below the processes for which it served. Of course, the fallacy of that is having poor ventilation in many basements. Machinery Room explosions have proven this point. Ammonia explosions can occur not only in basement Machinery Rooms, but also in any poorly ventilated Machinery Rooms, and particularly those with low ceilings. Ammonia gas wants to rise, so the Machinery Room, as a minimum, should be 25 feet high with very controlled air movement. Typically we would place multiple exhaust fans along one Machinery Room wall in the ceiling and locate all vessels and recirculator pumps below them. This would permit air pattern in one direction from one side of the Machinery Room to the other. The air inlets would be on the opposite end on the lower areas of the wall. Both the exhaust fans and the inlets need to be spread across the width of the Machinery Room.
Of course, in Northern climates, the fans need to be activated with thermostats and/or ammonia detectors to prevent freezing of the pipes and making the temperatures too low in the Machinery Room area. In some cases, heat may be required, but this should be electric as opposed to open flame gas.
With Machinery Rooms of reasonable height, our calculations indicate that even in the event of a liquid spill in the Machinery Room environment, the concentration ratios would be low enough for the ventilation fans to remove the ammonia gas before it reached explosive levels. Although there are many Machinery Rooms that have additions and multiple walls, generally the effort should be made to keep the Machinery Room as one rectangle. Machinery Rooms in the order of 60’ x 60’ can handle refrigeration capacities well into the 3,000 to 5,000 TR range. Judicial use of space, like using vertical recirculators, facilitate in good space utilization. More times than not condensers are located above the roof, which helps ventilation for these components and places most of the high side (pressure) refrigerant above the roofline.
Now with IIAR switching to volume basis (30 air changes per hour), green engineers are thinking low ceilings. So now we can start to blow them up and use high-risk horizontal recirculating vessels. I would hope we can get some latitude so design engineers can specify safe systems in all regions of the country and world.
Oh, one last thing we need to consider: the 30 air changes per hour doesn’t recognize the quantity of ammonia in the room as it has in the past. As shown in the following picture, you could possibly have a 30’ x 30’ machinery room with 60,000 pounds of ammonia.
The compressors for this portion of the system were (4) 800 HP diesel compressors which we do not want to place in the same room as the ammonia vessels. So, the point being, changing to cubic feet has caused many aberrations to safety. In this case, 30 air changes may not be enough. Hopefully, in the future we can refine the requirements for machinery room ventilation to recognize them. RR
To read the rest of the articles from the Spring 2015 Refrigeration Review, click the links below.