So, How Do You Keep Frost, Ice Out of Warehouses?
Many of the attendees came to take advantage of seminars discussing how to enter the new world of e-business. Software vendors were there to show how PRWs (public refrigerated warehouses) could make information such as ordering, shipping, and inventory available over the Internet to both customers and suppliers.
Other seminars revolved around food technology, food safety, government compliance, and other issues of concern for food industry professionals.
During a 4-hr forum, various exhibitors from the adjacent trade show discussed the different kinds of technologies they could provide that would help PRWs do their jobs better. These included better storage procedures, voice recognition technology used on the warehouse floor, and how to keep refrigerated floors clean.
Two sessions dealt with refrigeration issues. One discussed how to keep warm, moist air from infiltrating the refrigerated space through loading docks. The other discussed liquid desiccant cooling and dehumidification.
Both seminars addressed the issue from the perspective of keeping frost and ice out of the refrigerated warehouse space.
Keeping Infiltration to a MinimumHank Bonar, Bonar Engineer-ing, Jacksonville, FL, spoke about the problems of infiltration in refrigerated warehouses. He noted that infiltration, which usually occurs through the loading docks, sometimes accounts for 70% of the load in a refrigerated warehouse. That results in much higher cooling bills.
“Most docks are refrigerated; however, many warehouses don’t use reheat on the docks,” says Bonar. “The purpose of the reheat coils is to keep that air unit active on the dock throughout the year.
“We want it to run, because when it runs it takes advantage of the temperature of the refrigeration on the cooler side of the system. That removes the moisture at a much lower power cost than if you let it go in the freezer, where you have to remove the moisture with a two-stage system.”
Bonar added that while it only takes 144 Btu to freeze water, it takes 1,000 Btu to remove moisture from the air. That can be a substantial amount for the refrigeration system to remove. In addition to higher energy costs, infiltration can also cause moisture to freeze upon floors and doors, making warehouse operating conditions hazardous.
When moisture goes into the freezer, it exits as water. Bonar used various facts and figures to show that once moisture enters the warehouse, the energy cost to remove 1 gal of water is $1. He asserts that if that same gallon of water were removed on the loading dock through reheat, it would only cost $0.50/gal in energy costs.
“Refrigerated warehouses make anywhere from 8,000 to 10,000 gallons of water per year. That’s a lot of water,” says Bonar.
Bonar concluded by stressing the fact that refrigeration systems should be designed to take out moisture on the loading dock, so the refrigerated warehouse doesn’t have to. “If you design it this way, you’ll save energy, and you’ll keep the doorways clean.”
Desiccant Success StoryBill Griffiths, vice president engineering, Kathabar Inc., New Brunswick, NJ, started off his presentation by noting that a properly designed, built, and maintained warehouse is not going to have frost and ice problems, at least not to the extent that it’s going to hinder the operation of the plant.
However, there are many older or multistory refrigerated warehouses that have an insufficient or nonexistent vapor barrier, and in those cases, desiccant dehumidification may be the most cost-effective way to solve those problems.
A liquid desiccant dehumidification system is not going to save any warehouse operating utilities, notes Griffiths, but it is a way to solve some frost and ice problems in existing plants.
With a liquid desiccant dehumidification system, there are two units:
1. The conditioner unit takes in warm, moist air and cools and dries it using a coolant (ammonia or refrigerant). This cooled and dehumidified air is then released into the space.
2. The regenerator unit heats the desiccant (using low-pressure steam, hot water, natural gas, etc.), which drives the moisture out of the desiccant. The hot, wet air is then exhausted outside.
Griffiths points out that some of the benefits of a liquid desiccant dehumidification system in loading docks and staging areas include the following:
- Eliminates fog;
- Eliminates frost and ice on floors and ceilings;
- Provides additional cooling;
- Reduces or eliminates defrost cycles on air-handling units; and
- Reduces moisture infiltration into the freezer from the loading dock, thereby saving some energy.
In coolers and freezers, the benefits are similar; however, an additional benefit is that it has the potential to provide improved operating efficiencies for the refrigeration plant through increases in suction temperature.
Griffiths related the success story of the Newark Refrigerated Warehouse, Newark, NJ, an eight-story building constructed in 1925.
The building had problems with ice and frost in the elevator shafts due to the negative stack effect that occurred when air was dragged down out of the freezer. There was also a high moisture infiltration into the freezers for the same reason, the negative stack effect dragging outside air down the elevator shafts in the summer. Finally, there was a problem with a wet floor in the first-floor staging area of the building.
A liquid desiccant dehumidification system was installed, and president Jerry Von Dohlen is thrilled with the results. He noted that previously they had to use 10 or 12 tons of refrigeration in the elevator shafts to stop the ice formation, only to have the refrigeration system take the heat back out again.
“With this desiccant system, we took this very dry air and pumped it into the elevator shafts, so we didn’t have to heat the elevator shaft hardly at all any more. We eliminated 75% of the ice formation on our coils. We stopped all the icing in the elevator shaft, and we saved 10 to 12 tons of refrigeration by stopping heating of the elevator shafts.”
The year after the system was installed, the building was completely dry. There was no ice anywhere, and if the floor became wet, it evaporated almost immediately.
Von Dohlen notes that the system is not cheap. “The equipment is expensive, but we’re using it because we researched it. That’s because we’re focused on natural gas technology.
“We’re putting in generators and an ammonia absorber for waste heat use to subcool our Freon. We’re using desiccants, and we’re using engine-driven compressors. We’re trying to get rid of electricity completely, because it costs over $0.9/kWh in our area.”
Von Dohlen concluded by stating that warehouses that have problems with moisture should look into desiccant dehumidification, as it has worked well for him. He likes the system so well that he’s even building a new warehouse that will include the same kind of system.
He says that in the new building, a desiccant dehumidification system will allow him to get rid of defrosting altogether, and he estimates he’ll save close to 20% of his refrigeration tonnage and more than that in energy once the system is completed. The building should be completed in a year, and Von Dohlen says to check back to see how it’s working.