The cold storage operator has cut thousands of lbs. of ammonia from its refrigerated systems in two plants by questioning the need for so much of it.
Equipment in Henningsen Cold Storage's Salem, Ore., equipment room, from left: Frick compressor, subcooler (smaller white vessel) and low-pressure receiver (large white vessel)
Some people have a knack for challenging the status quo. Pete Lepschat, engineering services manager for Henningsen Cold Storage, is one such person.
The conventional wisdom in the cold storage industry has long been to use copious amounts of ammonia and not worry about it. Nowadays, there is a shift in thinking that is leading to lower ammonia charges in rooftop packaged refrigeration units as well as conventional engine room set-ups. At Henningsen, based in Hillsboro, Ore., outside of Portland, Lepschat began questioning the need for large ammonia charges more than seven years ago.
“I would be asked, ‘ammonia is cheap, so why use less?” said Lepschat. His answer: “Because it’s the right thing to do – to protect your employees and your surroundings. Ammonia is a great refrigerant, it’s natural, but it’s toxic. And as a fringe benefit, when you’re under 10,000 pounds, you are a little less subject to scrutiny from the regulatory agencies.”
That philosophy led Henningsen to install unorthodox ammonia refrigeration systems designed by Lepschat in two plants, with a third on the way. The systems, using an engine-room design, dramatically reduce ammonia charge as well as energy consumption compared to conventional designs.
Henningsen, a fifth-generation family-owned business founded in 1923, operates 10 cold storage facilities, including six clustered in the Pacific Northwest, and four scattered across the U.S. in Idaho, North Dakota, Pennsylvania and Oklahoma.
Keeping ammonia flowing
Henningsen has two warehouses in the Portland suburb of Gresham; the company first turned its attention to reducing ammonia charge at the second Gresham facility when it was built in 2009.
Lepschat's basic approach was to “look at every component [in the traditional refrigeration system] and ask if we need that much ammonia,” he said. “We took the system apart, eliminated a few pieces of equipment and got the charge down.”
Traditionally, condensed liquid ammonia from the condenser goes to a high-pressure receiver (HPR) tank, where it remains until called upon. But Lepschat asked, why store the ammonia? Why not keep it busy? “So we eliminated the tank and put in a small vessel [two-feet diameter, four-feet tall] that maintains a low level of ammonia.
““We took the system apart, eliminated a few pieces of equipment and got the charge down.”
– Pete Lepschat, Henningsen Cold Storage
The small vessel, essentially a way station, is fitted with a level probe and modulating Hansen valve; when the liquid level rises, the valve opens, feeding the ammonia directly to a low-pressure receiver (LPR) via a subcooler. Managed by a Logix control system, the overall design is called a “pump-down system.”
“So instead of storing a charge in the high-pressure vessel where it does nothing, we send it down to the low side right away. We put a little extra surge volume in our low-pressure vessel in a constant flow.” By taking out the HPR, Henningsen reduced the ammonia charge by several thousand pounds.
Lowering the feed
Another way to reduce charge is to lower the overfeed ratio of ammonia in the evaporators, which in a typical Henningsen facility is 3-1 (for every three parts of liquid ammonia, one part changes to gas).
In the Gresham facility, Henningsen purchased evaporator coils from Frick that enabled a 2-1 overfeed ratio. The lower overfeed ratio reduced the size of components like the LPR, the liquid piping and the coil tubes, decreasing the ammonia charge several thousand pounds more.
The combination of the lower overfeed ratio and the pump-down system cut the charge to about 5,000 lbs. from what would have been more than 10,000 lbs. “in most conventional design scenarios,” said Lepschat. The refrigeration capacity for that charge was 430 TR.
In 2014, Henningsen added about 50,000 square feet of freezer space as well as additional refrigeration equipment to the Gresham facility, increasing the total ammonia charge to 7,500 pounds and the capacity to about 630 TR.
Having successfully lowered the ammonia charge and energy consumption in its Gresham plant with a system that has “been stable and run well,” said Lepschat, he turned its attention to Henningsen’s first Salem, Ore., plant, built in 2013-2014. “That was my next chance with low-charge.”
At the Salem plant, Henningsen installed the same pump-down system used at the Gresham warehouse, but changed to Evapco evaporator coils with a 1.2-1 overfeed ratio. “That’s a really low rate; you have mostly gas coming back from the evaporators,” said Lepschat. “It’s the optimum ‘happy spot’ for an overfeed rate in terms of minimizing your charge and still having really good ammonia surface wetting on the coils and good heat transfer.” Moreover, the Evapco coils allowed Henningsen to use smaller liquid pipes and pumps.
The Salem warehouse also reduces ammonia charge by employing glycol rather than ammonia in a thermosyphon oil cooling process. A conventional system uses 2,000 to 6,000 pounds of ammonia just to cool the oil in the screw compressors. “That seems silly,” Lepschat said. Instead, in the Salem system glycol captures 360,000 BTU/hour of oil heat and channels it to the flooring underneath the freezers to keep them from freezing. This also saves energy in the condenser, which would ordinarily have to reject the oil heat.
In its second Salem cold storage facility, expected to open in 2017, Henningsen plans to leverage oil heat to reheat the air and thereby control the humidity in its docks. “We’ll bring moisture out of the docks before it gets to the freezer, which will reduce defrosting,” said Lepschat.
The new Salem plant, with -5°F freezers, will be similar to the original, though smaller (163,000 sq. ft.) with 3,000 lbs. or less of ammonia charge and about 250 TR of capacity. It will eventually add another 250 TR but maintain roughly the same charge.
To read the complete article about Henningsen Cold Storage, which appeared in Accelerate America’s November-December 2016 issue, click here.