Critical Flow

Borrowing from a previous publication with equations for the pressure drop of two phase flow, a method of calculating critical flow was included in the article. All designers should become familiar with this because this is what causes reverse flow, such as happened at Millard Refrigerated and several other plants that experienced low temperature evaporators being exposed to the piping system, that inadvertently reduced pressure because of cold evaporators and causes critical reverse flow. This is from the article referenced below:




(With Emphasis on Recirculation Pipelines)

By: Henry B. Bonar, II


Second Edition 02/15/1997

Originally Given At:



OCTOBER 24, 1996

The calculation for critical flow from the article is as follows.

Critical Flow

P2 – Downstream (lower) pressure

P1 – Upstream (higher) pressure


5  Chaddock, Werner, Papachristou, “Pressure Drop in the Suction Lines of Refrigerant Recirculation Systems,” Paper presented at ASHRAE Annual Meeting, Nassau, Bahamas, June 25-29, 1972. RR

To read the rest of the articles from the Spring 2015 Refrigeration Review, click the links below.

Questions for EPA

I was recently asked by a news journalist what questions I would ask EPA if I had the opportunity. Not to steal their thunder, but this is what I would ask.

Why are closed cycle refrigeration systems regulated under OSHA? Just because ammonia is a refrigerant does not make the refrigeration cycle a process by any chemical definition. The refrigerant does not change chemically in any way as it circulates inside the system. The type of concerns for safety in the PSM program are intended for a chemical process, i.e. Upset conditions, energy balance, etc. Ammonia has a very good safety record compared to other chemicals. The incident rate from refrigeration systems is way below incidents from other chemicals such as natural gases, which, if anything, should be a process because when it is burned and used for heating it is a process where the chemical is changed.

The next question is why is there so much concern about ammonia in our environment? Tons of ammonia is produced by lightening each year, and all forms of life contain it. It is a root chemical in our plant and animal structure. Farmers inject it directly in the ground as the “cleanest” fertilizer possible. It has no run-off side-effects, etc. It is truly green. RR

To read the rest of the articles from the Spring 2015 Refrigeration Review, click the links below.


Comments to CSB Regarding the Safety Bulletin about the Anhydrous Ammonia Release at Millard Refrigerated Services

The CSB issued a report on the Millard Refrigeration incident. In my opinion, it was incomplete, so I wrote comments to them, which I have listed below.

I reviewed your recent article about the Millard ammonia incident. While I applaud the efforts of improving safety, I would like to provide additional information that may shed new light on the probable causes of the incident. I was asked by one of the insurance carriers to investigate the incident. As designers of many hundreds of ammonia systems, we are attuned to the nature of two phase flow characteristic of closed central ammonia systems. The flow of liquid and gas in suction lines necessitates care in the design of the piping system. Primary of the design is to be sure the suction line slopes adequately from the evaporator (air units) to the machinery room recirculator, liquid separator, and vessel. These lines are quite lengthy in some facilities, which can be a challenge to be sure it slopes continuously. If liquid is permitted to collect or lay in the line, it can pose a hazard for more than one reason. If too much liquid is in the suction line it can possibly cause a liquid flood back to the machinery room and, depending on the gas velocity and rate of startup, can be more liquid to return than the recirculator can separate. Also, if critical pressures are created in the suction line, reverse flow can occur. Critical flow occurs when pressure in one place of the piping system gets approximately one half of pressure in another part of the suction line piping. Critical pressure is a well-known phenomenon that can be calculated for a given gas, temperature, etc. It normally has a 1/2 relationship. What is important about this condition is once it occurs, gas from the higher pressure area will move toward the lower pressure area very rapidly and try to approach sonic speed. In refrigerants this is very critical because when it occurs, the pressure “head” will make any gas ahead of this gas condense instantly and change from gas to liquid. This is what I have called liquid cannon balls. The Millard system had two, maybe three, things going against it. The first was the slope of the pipe line. While the industry says 1″ in 40 ft is OK we have always used 1″ in 20′ for the reasons outlined. 1″ in 40′ is very hard to maintain. When asked to inspect the Millard system, the first thing we did was have the pipe line surveyed for elevation over its entire length. We found two areas that were trapped as much as 2+ inches. This, of course, leaves large pockets of liquid. The second was the use of what is known as CK-2 suction stop valves. These valves are a little cheaper than their counter part S9As but the CK-2 fails open not closed. We have seen this type of failure occur on other systems using CK-2. Why this is significant is that if, for any reason the CK-2s (which are used in each blast freezer defrost control group) drop open, low pressure from hot gas supply (which keeps the valves closed), interrupted defrost etc., the pressure in the evaporator coil is going to drop to the corresponding saturated temperature of the blast freezer coil. If that pressure is half the pressure that is in the pipe line near the machinery room, then reverse sonic flow will occur. Quite often when this occurs the liquid “cannon ball” will not only break open the end of the pipe line, it will enter the air unit coil with enough velocity to rupture the coil header, which I understand also occurred. So my recommendation would be to add the following:

  • Slope suction lines 1″/20′ and be sure they are sloped continuously, with no traps
  • Use fail close suction stop valves (S9As), not the cheaper CK-2 valves.

Hope this will add clarity. RR

To read the rest of the articles from the Spring 2015 Refrigeration Review, click the links below.

Machinery Room Ventilation

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.

test1232 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.