RANDOLPH, N.J. — Cooling towers are a relatively inexpensive and dependable means of removing heat from drycleaning machines (DCM). There are other ways to cool your equipment, but if you are paying for electricity and water, a cooling tower is often the most economical.
Cooling towers use the outside air to cool water that is circulated through the DCM’s refrigeration heat exchangers, still condensers and/or solvent coolers.
In an evaporative cooling tower, some of the water being cooled evaporates into a moving air stream. When water evaporates, it removes lots of heat from the rest of the water. Each one pound of water that is evaporated removes approximately 1,000 Btu from the system.
Water lost through evaporation is replaced by what is called “makeup” (city water). Removing heat by evaporation is highly efficient. That’s why you can cool a large amount of water with only a small amount of evaporation.
COOLING TOWER SYSTEM
A “cooling tower system” might seem complicated, but it’s not.
An electric pump takes the water from the storage tank and pushes it through the coils in the DCM. The water picks up heat from the coils and then flows out of the DCM and up to the cooling tower.
It goes up to the top of the cooling tower and flows through the tower from top to bottom. The water is cooled. Cool water then drains out of the bottom of the tower through a large pipe to a storage tank where the pump starts the cycle over again.
Depending on the size of the tower, this water flow can be 75-300 gallons per minute (GPM). Most of the water is reused at 93%.
The key to tower performance is rapid water flow. A cooling tower doesn’t drop the water temperature as much as a refrigeration unit, so the higher flow makes up the difference.
When the water flow is too low, the water does not flow through the tower evenly. When that happens, there is not enough evaporation. Without evaporation, the water is not cooled enough.
Now you have two problems: the water is not flowing through the coils in the DCM fast enough, and the temperature of the water flowing into the DCM is too hot.
Both of these issues can cause the DCM to malfunction. Evaporative cooling towers are highly dependable. However, when there is a problem, it’s normally caused by reduced water flow.
To understand water flow, it’s important to understand the water pumps used in towers. Most of these pumps are “forward vane centrifugal pumps.”
Tower pumps are similar in function to the solvent pump in the DCM. When there is too much restriction to flow, these types of pumps simply pump less water. The pumps are still turning the same RPM and there is no buildup in pressure, but they pump less.
Water flow restrictions in cooling tower systems are the resistance to pumping water through the coils and the energy to pump the water up to where the tower is located (on the roof). This combined resistance is called “discharge head.”
When cooling tower systems are selected, the installer is trying to select a pump that will pump enough flow that allows the tower to cool the DCMs. Unfortunately, there are a lot of ways that can go wrong.
For example, a tower system might be sized to cool three DCMs. The pump can overcome the discharge head of the coils in the three machines as well as the vertical lift to the tower on the roof.
But then let’s say, two of the machines get turned off. Now all of the water is trying to get through one set of coils.
The discharge head is too high for the pump, the water flow slows down, distribution in the tower is affected, the water temperature goes up, and the one operating DCM shuts down for lack of water.
The operator thinks, “How could the tower cool all three machines running at the same time, but can’t cool one machine running solo?” So, he switches to city water and curses the mechanic who sold him the cooling tower system.
Other things can cause the discharge head to change. Many modern DCMs have valves that limit or shut off the water flow through the coils when the coils are not in use, such as the refrigeration condenser during the wash cycle.
Another big user of water is the “still condenser,” which only needs water when the still is operating. These are all good devices to save water if you are using city water for cooling. Unfortunately, they work just the opposite when the cooling tower system is used.
The adjustment for this is the “bypass system.”
Properly installed cooling tower systems will have piping that connects the cooled water pumped from the storage tank (inlet header) to the warm water going back to the tower (outlet header), bypassing the DCMs.
A valve should be installed in this pipe. If the discharge head is too much for the pump, the operator slowly opens the bypass valve, which allows some of the cooled water to bypass the DCMs and go straight to the tower.
This increases the flow and improves the distribution in the tower, which lowers the water temperature—the dry cleaner is back in business.
Care should be used when opening the bypass valve. It should be opened just enough to improve tower operation.
If it is opened too much, the DCM will not get enough flow for cooling. This is especially helpful during high-temperature, high-humidity days.
The author acknowledges that much of the information in this article was supplied by Scott Pariser, Pariser Industries Inc.
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