Defining World-Class Refrigeration System Design
According to American Heritage Dictionary of the English Language, Fourth Edition, world-class is defined as follows:
world-class (wûrld´klas´) adj. 1. Ranking among the foremost in the world; of an international standard of excellence; of the highest order: a world-class figure skater. 2. Great, as in importance, concern or notoriety.
What defines a world-class ammonia refrigeration system? I have heard this question many times. Of course, the definition is different for every company because it depends on the company's needs and applications, but the truth is that very few end users actually sit down and try to define what "world class" means to them.
Every aspect of the refrigeration system can be defined in terms of how it meets world-class criteria. This article briefly examines three areas: safety, quality control during construction and insulation.
Design for Safety
One of the most important steps is to design the machine room to hold the majority of the refrigerant and be able to contain it in the event of a release. Receivers should be designed to carry the entire refrigerant charge and located inside the machine room for total containment.Solenoid and check valves should be located strategically throughout the system to isolate areas for refrigerant containment in the case of a release. Pipe all relief valves to a properly designed central header, which is monitored for a release and controls and neutralizes the release by one of three methods (The method used depends on code requirements.):
- Release into a vessel of water at the rate of one gallon of water for each pound of ammonia in the system. This is the safest method.
- Release into a mixing valve with water at a rate of two gallons of water per pound of ammonia. The neutralized mixture then is dumped to either a retention pond or to a process sewer.
- Release to a natural gas flare.
You also will want to install relief indicators at each relief valve to identify specifically which valve was relieved. This also can be wired to the control system for annunciation.
Monitor all rooms for leaks and have the system designed to automatically take the appropriate actions such as sounding alarms, paging operators and turning on exhaust fans. At higher levels, the system should be able to automatically call the fire department and shut down all electricity to the machine room.
Duct the machine room exhaust fans to an ammonia scrubber, which typically is located on the roof. Develop a plan to remove or contain contaminated water from the scrubber during a release, or have permission to dump it to the process sewer. Form containment areas in the machine room using a combination of dikes, floor drains and pits to contain the release in the event of a liquid spill. This design facilitates total containment of a release in the machine room whether the release is liquid or vapor.
Where possible, all pipes and valves should be installed on the roof out of refrigerated rooms. Pipe stands should be designed for worst-case wind and seismic conditions. In most cases, some connection to the roof structure is required. In all cases, the pipe should be secured to the pipe stands.
Be sure to include a pump-out system throughout plant. This allows refrigerant to be removed from an area of the system safely without having to endanger operators or vent to atmosphere.
Another safety precaution to take that ensures designing a world-class ammonia refrigeration system includes minimizing refrigerant charge where feasible. Examples include:
- Use critical charge systems.
- Use plate-and-frame condensers and cooling towers to remove heat of rejection.
- Use plate-and-frame heat exchangers instead of shell-and-tube to reduce charge.
- Use glycol as a secondary refrigerant, especially in process air-conditioning applications with a lot of personnel.
Quality Control During Construction
To ensure quality control during the construction of your ammonia refrigeration system, the first thing to do is require that the contractor submit the welding procedure to be used on the project and ensure that all welders are certified to perform that specific welding procedure. For a world-class system, require a tungsten inert gas (TIG) root welding procedure. This procedure actually generates the weld in an inert gas environment that prevents slag buildup inside the pipe. This provides a clean weld on the inside and minimizes contaminants in the system.Weld maps should be required for every project, with every weld identified and the welder that performed it recorded. Every weld should be stenciled with the welder's identification.
Also, require at least 10 percent nondestructive testing (either radiography or ultrasonic) of all welds during the construction process by an independent agency. Refrigeration piping code B31.5 only requires visual inspection of the welds and does not define criteria for doing radiographic inspections; therefore, it is imperative to require welding be performed to code B31.3, which does. The weld inspections should be distributed so that a minimum of 10 percent of each welder's work is inspected. Inspections should continue during the entire duration of the project -- not all at the beginning, all at the end or even worse, all done on one welder.
Require all pipe to be cleaned, primed and painted before insulation. This is the best way to prevent corrosion from occurring under the insulation where the system is out of site. Be sure that all pipe is delivered capped, and while on site, require pipe to be kept clean and stored dry and off of the ground.
Insulation Ensures Quality
Controlling vapor intrusion into the system is the primary ingredient to longevity of the insulating system. This takes several layers of protection. It is important to insulate the system when it is dry. If the insulation has to be left open during construction for any length of time, cover it.Specify the ambient condition that the insulation will operate in as the absolute worst case, not the worst average. Insulation thickness is designed to prevent condensation on the surface at a certain ambient wet bulb temperature. If the wet bulb exceeds this condition, condensation will form on the vapor barrier, under the jacketing, which is tough to fight.
Anything more than 2.5" thick should use a double layer. The inside layer should not be banded or caulked, and the outside layer should have a complete layer of caulk (not just a bead) across the entire face of the joints. The caulk seals the seams between the insulation pieces to prevent moisture intrusion. The joints should be staggered and offset.
At all valves and places where the insulation will be disturbed during operation or maintenance, have vapor stops installed on both sides to prevent moisture migration. During regular maintenance, have places where valve stems and pipe penetrate the insulation re-caulked. This should be done at least every six months.
You also will want expansion joints installed at least every 100' on long straight runs, especially at elbows and tees where they change direction. Vapor stops should be installed at least every 100' so if there is a breach in the vapor barrier, it limits the contamination.
Install the absolute best vapor barrier available. Polyguard, Saran and Mylar are good examples. These products have superior permeability to traditional solutions like all-service jacketing (ASJ). When the vapor barrier is installed, it should not have any holes or voids.
Jacketing also is important. Aluminum jacketing has become the standard for outdoor applications. There has been a significant problem in the industry recently with the aluminum jacketing corroding from the inside. This has been addressed by using a jacketing with a polymer-based backing rather than paper-based backing. Stainless steel jacketing is becoming more prevalent and is the best solution. Finally, use 30 mil PVC jacketing inside.
If you follow these guidelines when designing your next ammonia refrigeration system, you will be on your way to one that is world class.
Links
