Wet cooling towers vs. dry coolers

Wet cooling towers vs. dry coolers

Cooling towers are devices that use heat transfer to send heat loads into the air. Cooling towers are used to cool water that is circulating in a wide range of systems, like process cooling and HVAC.

Cooling towers get rid of heat by sending it into the air when a stream of water cools down. Cooling towers come in two main types:

Open-circuit cooling towers, which are also called "wet cooling towers" because the process water mixes directly with the air around the tower;

In closed-circuit cooling towers, also called "dry coolers," the fluid being cooled does not come into direct contact with the outside air.

Both work to cool the water, but they do it in different ways.

Wet cooling tower uses the latent heat of water evaporation to exchange heat with the air stream that flows through it. It can bring the process water temperature down below the ambient temperature to a certain degree above the dew point (approach temperature). In contrast, a dry cooler only brings the water temperature down to the ambient temperature. However, the inlet air can be pre-cooled by adiabatic means to cool it even more. Wet cooling towers work well in hot, dry climates, but when the air around them gets more humid, they don't work as well. On the other hand, a dry cooler's performance is not affected by how wet the air is. Instead, it depends on how hot the air is.

System Design Dry Coolers

Dry coolers are easy to use and built to last. They have a heat exchanger, which is usually a finned tube or microchannel coils, and fans that push air through the heat exchanger to cool the water or glycol solution that goes through it. Most dry coolers have airflow that is either horizontal or vertical. Most dry coolers can reject heat up to 1MW, but there are also large-capacity dry coolers on the market.

Modern designs may use either adiabatic spray or wetted pads as a pre-cooling system. With evaporative pre-cooling, the process fluid can be cooled below the dry bulb temperature.

Cooling Towers That Get Wet

A wet cooling tower is built in a much more complicated way. It has nozzles that spray hot water evenly onto the heat-exchanging media (also called "fill" or "wet deck"), fans that move air through the cooling tower, "drift eliminators" that stop water droplets from leaving with the air, and a basin for collecting the cooled water. Air flows from the bottom of the tower (counterflow type) or in the opposite direction of water flow (perpendicular flow type) (crossflow-type tower).

Film fill and splash fill are the most common types of heat exchange media. Film fill is a set of textured sheets with a large surface area. When water hits the airflow, it spreads out and forms a thin film. This makes sure that water and air can exchange heat quickly. Splash fill is a structure with many layers that makes small droplets of water and air come into more contact with each other. This makes for a high heat transfer rate. The type of fill media should be chosen based on the quality of the water. Film fill is best for clean water, while splash fill is best for dirty water with a lot of solids.

There are special ways to stop bacteria from growing, like anti-legionella fill media, which works by stopping microorganisms' metabolism and stopping their ability to grow and reproduce.

Drift eliminators are an important part of making sure a cooling tower works well and doesn't lose too much water. A drift eliminator is built so that the airflow is deflected more than once. This makes water droplets carried by the airflow land on the surface of the eliminator and drip into the collection basin.

For and against

Wet (open-circuit) cooling towers are better for big factories that need to cool down a lot of water:

Pros:

Ability to cool process water below the dry bulb temperature; Reliable operation in hot environments; High cooling capacity per unit.

Cons:

High costs for equipment and installation; high costs for running the system, which include treating and refilling water often, maintaining system parts, and paying for electricity;

Due to evaporation, the plants use a lot of water;

Rust is forming along the pipes;

Legionella and other bacteria grow.

Dry coolers are best for small to medium-sized commercial and industrial applications, as well as free cooling systems:

Pros:

Low start-up, running, and maintenance costs; Can work in any weather, from hot and humid to very cold;

Wide range of unit designs that make it possible to use it in almost any environment and for almost any purpose;

Fewer clogs in the system and less contamination of the process water.

No loss of water.

Cons:

Can't cool the process water below the temperature of the dry bulb (without adiabatic pre-cooling); Low capacity per unit.

As closed-circuit systems, dry coolers have several advantages over wet cooling towers, such as lower initial costs and maintenance costs, better system reliability, and more time when the system is actually working. Since dry coolers have evaporative pre-cooling, their efficiency is only limited by the wet bulb temperature, just like wet cooling towers.

In light of all of this, the system capacity is still the most important thing to think about when choosing between an open-circuit cooling tower and a dry cooler. With the constant development of more efficient technologies like microchannel heat exchangers, EC fans, intelligent controls, and others, it is possible to think of dry coolers as a useful part of new thermal systems that are better for the environment.