Closed-circuit Industrial Coolers
What is Closed-circuit Industrial Coolers?
Closed-circuit industrial coolers, also known as evaporative fluid coolers or closed-loop cooling towers, are cooling machines that use evaporation to lower the temperature of a main water mass. They use a closed loop design to keep the system clean and contaminant-free, and to prevent direct contact between the HVAC or industrial process fluid and the ambient air.
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When we talk about cooling machines, we are used to think about the typical open-circuit cooling tower, an evaporative cooler operating through a direct contact between air and water. Nevertheless, this is not always the best solution for civil and industrial plants: in some projects it could be better to avoid a direct contact between the air and the water (hydraulic separation) and, therefore, a closed-circuit cooling tower can be preferred.
Now we know the basic elements and we can better understand the 3 advantages of closed-circuit cooling towers.
No contamination of the primary circuit. Many civil and industrial processes require to maintain the process fluid uncontaminated by the external environment: for instance, this is true for most of food production processes. Closed-circuit cooling towers, grant this need: in effect, coils separate process fluid from ambient air drawn in by the fans, keeping it clean and uncontaminated in a closed loop.
Reduction of freezing risk. Since the process water circulates in heat exchange coils and never gets in contact with air, glycole can be added to it to lower the fluid freezing point. This can be a big advantage especially in cold countries (in e.g. Scandinavian countries, Russia, Canada, …) where temperature frequently goes below zero.
A simpler system. A closed-circuit cooling tower can be an alternative to a system composed by an open circuit cooling tower and a heat exchanger. This leads to some advantage:
- Easier plant layout
- Reduced footprint
- Higher thermal efficiency
Working Principles of a Closed Circle Cooling Tower
The closed-circuit cooling tower is designed to cool water at the lowest possible cost in two ways, dry and wet. In the dry method, the environment to be cooled is cooled to ambient temperature, while in the wet method, the humidity conditions are manipulated to provide cooler air than the ambient temperature. In this system, cold air cools the environment and as a result, energy consumption is saved.
Cooling towers are often used in residential and commercial areas such as the roofs of hospitals or shopping malls because they work quietly thanks to their radial design.
Closed Circle Cooling Tower Operation
Closed-circuit cooling towers operate similarly to open-circuit cooling towers, except that the heat load to be passed through the process fluid (refrigerant) is transferred to the ambient air through the heat exchange coil. The coil works to separate the process fluid from the outside air, keeping it clean and free of any contamination in a closed loop. It creates two separate fluid circuits:
An external circuit in which water is sprayed around the coil and mixes with the outside air and an internal circuit in which the process fluid circulates inside the coil. During operation, heat is transferred from the internal circuit through the coil to the spray water and then transferred by evaporation of some water to the atmosphere.
The closed circle tower system consists of an open cooling tower with a separate heat exchanger package. The hot side of the heat exchanger also never comes into direct contact with air. This allows the heat exchanger, along with other indoor system benefits, to be protected from outside weather conditions. This closed system is much more durable than the external winding type closed circle cooling tower.
There are three main types of closed-loop cooling systems: closed-loop dry systems, closed-loop evaporative systems, and liquid-to-liquid cooling systems.
Closed-Loop Dry (Air-Cooled) Systems. This type of closed-loop system utilizes an air-cooled heat exchanger to release the heat absorbed by the coolant into the atmosphere.
Closed-Loop Evaporative Systems. This cooling system employs an evaporative type of heat exchanger and evaporation to remove the heat that has been absorbed by the coolant.
Liquid-to-Liquid Cooling Systems. As its name would suggest, liquid-to-liquid cooling systems use heat exchangers to transfer the heat from one coolant to another; typically once the coolant has circulated through the process, it travels to the heat exchanger and then the heat is transferred to the liquid in a cooling tower, body of water, etc.
Closed Circuit vs Open Circuit Cooling Towers
They both provide heat rejection but in slightly different ways. These systems are often referred to as open and closed loop systems. These are the two commonly used HVAC cooling tower system designs.
Closed-Loop vs Open-Loop Operation
In a closed-circuit cooling tower, the process fluid, which could be water, or a water-glycol mixture is circulated within a closed loop piping system. There are two separate water sources, one external within a closed loop, and the second one that circulates water from the tower basin over the heat exchanger. There is never any contact between the water in the enclosed loop and the water circulated within the tower.
The water in the tower basin is pumped up and sprayed over the coil that is fed from the closed-loop piping while a fan forces air over the wet coil. This increases heat transfer through the coil while minimizing the evaporation of water from the basin. The cooled water in the closed-loop coil returns to the building to absorb more heat.
Return air brings heat from the space over the indoor coil. The refrigerant cycle moves that heat to the water-cooled condenser coil, where the water circulated from the cooling tower picks up that heat. The heat is pumped to the cooling towers closed-loop heat exchanger coil where water is sprayed over it as air is induced or forced over the coil.
Open-Circuit Cooling Tower
In an open circuit cooling tower, the water is directly exposed to the outside air. Water enters the top of the tower and is sprayed over the fill or heat transfer media. The exchange of heat occurs directly between the water and the entering ambient air. The water that is circulated to the chiller's condenser and the air within the tower touch each other. This increases the chance of contaminants being scrubbed out of the air and into the cooling tower basin.
The water in the basin is then returned to the facility or the condenser side of a water-cooled chiller, which could foul the chillers condenser coil if the water is not properly treated. The makeup water to the tower can also introduce contaminants to the process water.
Induced-Draft vs Forced Draft
There are several tower configurations including induced draft and forced draft. The fans for an Induced draft tower are located at the top of the tower and induces air into the tower. The fans for a forced-draft tower are located at the bottom of the tower and forces air into the tower and over the coil.
Closed Circuit Cooling Tower Advantages and Disadvantages
Closed circuit cooling towers also known as Evaporative Fluid Coolers play a crucial role in the operation of water source heat pump systems in the HVAC industry. It's important to use a closed loop system because the water is circulated into the coils of all the water-source heat pumps scattered throughout the building. Sending water from an open tower into all these remote coils could create a maintenance nightmare along with a reduction in efficiency for these units.
Advantages of a Closed-Circuit Cooling Tower
Reduced risk of contamination since the process fluid is isolated from external elements.
Water conservation as the process fluid is recirculated within a closed loop.
Less susceptibility to scaling and fouling due to the closed nature of the system.
Reduced equipment maintenance.
Reduced water treatment cost due to lower volume of water circulated from the basin.
Sensible heat rejection with pumps off, saving additional energy when conditions are right.
Disadvantages of a Closed-Circuit Cooling Tower
Typically, higher initial cost due to the need for additional equipment like pumps and heat exchangers.
Requires maintenance of the closed loop system to prevent corrosion and ensure fluid quality.
Applications
Closed circuit cooling towers are often preferred in applications where water quality is critical, and there's a need to minimize the risk of contamination. This makes them suitable for water-cooled heat pump applications and those industries with strict quality standards, such as laboratories or data centers.
Advantages of an Open-Circuit Cooling Tower
Generally lower initial cost as there is no need for additional closed loop equipment.
Simplicity in design and operation.
Large range of capacities and configurations.
Energy efficiency.
Disadvantages of an Open-Circuit Cooling Tower
Greater susceptibility to contamination from external elements like dust, debris, and biological growth.
More water consumption as water is continuously replenished and not recirculated.
The requirement for water treatment.
Extra level of maintenance.
Reduced efficiency due to the scale and/or fouling of the Chillers Condenser coils if water treatment is not properly maintained.
Complicated design when tower is installed below the piping system or pump.
Applications
Open circuit cooling towers are commonly used in the HVAC industry where water quality is less critical, and the focus is on cost-effective cooling. They are found in all types of buildings spanning the HVAC industry. The addition of a heat exchanger between the cooling tower and the chiller can add a layer of protection against the fouling of the chiller's condenser tubes, but at an added cost. This doesn't eliminate the need for water treatment of the tower, it just shifts the tower basin water from contacting the chillers coil to the heat exchanger.
Closed Circuit Coolers: Design With Winter In Mind
When outside temperatures fall below freezing, water inevitably forms into ice. Open cooling towers can be susceptible to freezing due to their inherent nature of operation. As a result, ice formation becomes a subject of management, not elimination. Fortunately, there are alternative solutions.
Closed circuit coolers are used for a multitude of niche applications, but are ideal for evaporative cooling applications in severe winter climates. If ice formation is not managed properly, expect to see reduced thermal performance, part failures, and additional field maintenance. Make sure that your closed circuit cooler is designed, specified, and built with winter in mind.
Selection:
For the ideal solution, select a closed circuit cooler with finned coils. This will increase your ambient dry bulb switchover temperature and allow the closed circuit cooler to run dry (spray pump off) for the maximum amount of hours during the spring, fall and winter.
Select a closed circuit cooler for optimum free cooling operation. Winter cooling conditions are typically more difficult to achieve than summer conditions and therefore should be the basis for equipment selection.
Implement an automatic draining system for the cold water basin when operating dry for extended periods of time. This will eliminate unnecessary and excessive operation of the electric basin heaters.
Process Fluid
Depending on your climate, utilize a solution of ethylene or propylene glycol in order to protect the process fluid during freezing conditions.
Remember glycol is less efficient at transferring heat then water. Factoring in derates is imperative while optimizing for the unit's capacity.
Fluid in motion is less susceptible to freezing. Adhere to the manufacturer's recommendations for minimum flow rates for freeze protection, especially when the process fluid is water.
A 30% solution of ethylene or propylene glycol protects the process fluid from freezing down to 6.7°F (-14°C) and 9.2°F (-12.7°C) when crystallization begins. The burst point of these two solutions is not until -60°F (-51°C)!
Accessories
Add a discharge hood with dampers and insulation to the casing section when the coil is located in the air stream to reduce heat loss. As an additional measure add insulation to the discharge hood with dampers.
Mechanical float valves are located inside of the unit and cannot be heat traced, leaving the valve susceptible to freezing. Substitute the float assemblies with an electronic water level control, and utilize an external solenoid valve which can be heat traced and insulated.
Add a heater package in order to keep your basin water from freezing. Size the heaters based on extreme ambient temperatures. Heaters are designed to maintain 40°F basin water temperatures.
Ice formation can occur on fan blades when inclement weather conditions present themselves. Include a vibration cut out switch to detect excessive vibration and shut down the fan motor before extreme damage can occur.
Include a "de-icing” control sequence which reverses fan direction for a set period of time. This technique will assist in ice management.
Our Factory
Shandong Hexinda Electric Furnace Co., Ltd. was established in 2014. After 10 years of development, the company has become a production-oriented company that integrates research and development, production, sales, and after-sales service, and has passed ISO9001 quality system certification. We produce a complete set of equipment, induction power supply, furnace body, and cooler for 100kw-20000kw medium frequency induction melting furnaces and induction heating furnaces, which can meet the casting, forging, and heat treatment needs of metals in different fields. The company not only has industry-leading technological level, but also has a complete pre-sales and after-sales service process.
FAQ
We're well-known as one of the leading closed-circuit industrial coolers manufacturers and suppliers in China. If you're going to buy customized closed-circuit industrial coolers, welcome to get more information from our factory.
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