To Stay Competitive, “Do the Math and Trust the Science”

Reduced-charge central ammonia refrigeration systems do not have to mean lower efficiency. With current technology, industrial refrigeration systems can be designed with a smaller ammonia charge and still operate at high efficiency. The key is trusting the math when designing a system.

“My analogy is that you don’t use the same computer that you did in 1988,” said Peter Lepschat, corporate manager of engineering services at Henningsen Cold Storage, Co., where he works on the design and construction of new facilities. “It’s the same thing with refrigeration. Control and design technology have advanced to the point where there are many things you can do now that you couldn’t just a few years ago. From a contractor’s perspective, customers like us are going to demand that you learn this new technology. If you don’t, at some point you will go out of business.”

An example of advanced technology is the evolution of the DX coil. “Previously, DX coils for freezers did not work well,” Lepschat said. “The coil designers didn’t have a [good idea of] how to design a coil with the right tube sizes and circuiting for DX use.” The expansion valves did not provide reliable operation. “But with advanced modeling software that now can be used, [along with new valve and control technology] you can scientifically design a coil that will work in direct expansion mode.”

DX coils are a great way to reduce charge, eliminating the low-pressure receiver, pumps and larger liquid ammonia piping used with overfeed evaporators.

When designing a system, Lepschat also recommends rejecting traditional rules of thumb. “The industry rule has always been that you need one ton of refrigeration capacity for every 400 square feet of freezer” he said. “That was probably true in 1955 when walls were insulated with sawdust or cork, you had slow-operating doors and highly inefficient lighting. But people still use that number. The result is a system that is way oversized. It’s all wasted ammonia. Do the math and trust the science to figure out what you really need.”

Lepschat recommends hiring an engineering consultant who is not financially invested in the facility to help with the design. “There is no incentive for contractors to lower [the system] charge,” he said. “They figure that if something has always worked, why mess with it? You need an engineer who is willing to ‘right-size’ components and is not going to put in additional margins that are not needed. Designing systems to be the right size has no effect on efficiency. It just removes ammonia from that system that is serving no useful purpose.”

One simple approach Lepschat takes is to consider liquid refrigerant as the enemy. Managing the liquid level in an ammonia vessel used to be more difficult, due to the use of fixed-float switches. But with the advent of electronic level probes that monitor liquid levels, and control systems allowing easy level adjustment, the levels in vessels can be fine-tuned to the minimum safe and stable operating level, typically allowing for a reduction in ammonia charge. “You might be able to get rid of a couple of tons of ammonia that was just taking up space in your vessels,” he said.

Another area that can provide opportunity for sizable reductions in ammonia charge is the liquid piping of a system. It is very important to size piping properly. Many systems have grossly upsized liquid lines. It is not unusual for a designer to install three-inch liquid pipes when two-inch pipes would be sufficient. “There is an old saying in the industry that pipe is cheap. If you’ve done the math and a two-inch pipe works from an engineering standpoint, use the science and size it right,” Lepschat said.

Another methodology Lepschat uses is to put yourself inside the system when designing it. “For example, if you have a low-pressure receiver, consider its purpose,” he said. “It contains liquid in the bottom, and there’s a pump sending it to the evaporators. The ammonia then comes back as a gas/liquid mixture. It separates and returns the gas to the compressor, and the liquid goes back to the pump. So, you’ve got all this liquid ammonia, but what is it doing? Well, the liquid is feeding the pump and the vessel is maintained to keep the pumping liquid going to the freezer evaporators. Ask yourself, how can I get the same effect with less ammonia?”

One way is to minimize the liquid inventory by holding the operating liquid level at or near the bottom of the vessel, so that the much smaller-diameter drop leg contains all the ammonia feeding the pump. This may require extending the drop leg to make sure you maintain NPSH requirements at the pump inlet, but the charge reduction would be significant.

“These are only a couple of examples of low-hanging fruit that we have found,” Lepschat said. “In reality, every component of your system offers some opportunity to reduce charge. You just need to take the time and ask the questions.”

In the final analysis, low-charge systems eliminate wasted ammonia, while maintaining efficiency. “The technology is there, and that’s where the industry is headed. Everyone needs to get on the train,” Lepschat said.