What should you know about choosing a dry cooler?

What should you know about choosing a dry cooler?

Dry coolers are heat rejection devices used to cool process liquids such as water, glycols, and oils by forcing heat energy into the atmosphere. Dry coolers, unlike wet cooling towers, are closed-circuit systems with no direct contact between air and process fluid.

Dry coolers can be used as standalone fluid coolers or as part of an air conditioning or refrigeration system to cool fluids in intermediate heat exchangers, water chiller evaporators, and for a variety of other applications.

Flatbed units with vertical or horizontal airflow directions are common, as are V-shaped units with one or two rows of axial fans and lateral air suction and discharge from the top.

Dry coolers are relatively simple appliances that are used in a wide range of industrial and commercial applications, from chemical plants to data centres and shopping malls. They are made up of two major component parts: air-to-liquid heat exchangers and fan blowers. Typically, axial fans are used, and heat exchangers are finned pack coils or, for small or medium systems, microchannel coils.

Dry cooler selection appears simple, and it is – until modern customer requirements such as optimised power consumption, reduced sound emission, or size constraints are carefully considered. Given the abundance of technical solutions for fans, heat exchangers, and assisting components like adiabatic pre-cooling, the task becomes even more difficult. What factors should be considered when selecting a dry cooler for a modern and efficient cooling plant?

Baselines

The basic input for the selection process includes fluid inlet temperature, outlet fluid temperature (or flow rate), type of fluid to cool, and ambient air temperature. However, a number of other parameters and conditions should be considered:

Collaborative unit placement. Closely spaced units will most likely perform worse. Drawing of hot exhausted air by nearby units should be avoided by providing sufficient distances between units or locating them on a gantry or frame of sufficient height. In the latter case, small gaps between dry coolers must be covered with blind panels.

Wind speed and direction. Because strong winds can have a significant impact on performance, the installation site may need to be fenced, which may also isolate unit-borne noise.

The installation site's elevation above sea level influences fan performance.

Fans must be chosen to withstand the temperature of the air exhausted from heat exchangers.

Working fluid must contain antifreeze in sufficient concentration to prevent the freezing of heat exchangers and pipework when operating in low ambient temperatures. It is recommended that AC-driven fans be used at very low ambient temperatures.

Fan noise. A good practise is to use EC-driven fans to reduce noise output, for example, at night, by limiting fan rotation speed. The soundproof fence is another effective method of noise reduction for the entire cooling plant. The soundproof fence is another effective method of noise reduction for the entire cooling plant.

When choosing a fan motor, consider expected daily/seasonal changes in cooling demand. For example, using EC-driven fans at partial loads can save up to 70% more energy than units with AC-motor fans.

Various fan speed controls and regulating signals are available. Units may have their own independent fluid temperature sensors and fan speed controllers, or they may be incorporated in higher-level systems that receive regulating signals from networked control devices.

It may be necessary to add dry cooler controls to the upper-level BMS system. As a result, the type of network protocol should be considered when selecting control options.

Pressure drop on the available water side. Excessive pressure drop may necessitate an increase in pump size and power draw.

The electrical/control panel is normally located directly on the unit; however, in some cases, such as installations in cold climates, it is required to locate an electrical enclosure remotely, indoors, or using a stand-alone weatherproof structure.

Individual switch-offs on fan blowers allow for hot swapping or maintenance without requiring a unit shutdown.

The fluid connection type.

Vibration dampers of various kinds.

Process Fluids

Water, water-glycol mixtures, and oils/special fluids are examples of typical process liquids. Various inhibitors should be used to prevent heat exchanger corrosion in specific cases, such as when fluid may contain contaminants or its pH level is outside of the allowable (safe) range. In the first case, a strainer must be installed. Some fluids may necessitate special protective heat exchanger inner coatings.

Even if the process liquid temperature is high enough to operate safely in the winter, antifreeze addition may be necessary to avoid freezing in the event of failures, blackouts, fluid supply shortages, or emergency repairs.

External pipework must include air vents to prevent oxidation, cavitation, and airlocks.

Fan Speed Control

Fan speed controllers maintain the fluid outlet temperature setpoint for dry coolers under any load by varying motor speed based on the sensed fluid temperature. Fan regulation is critical because dry coolers are designed for maximum conditions - maximum ambient temperature and thermal load. In practise, maximum load combined with maximum ambient temperature occurs for a very limited time of year, making dry coolers oversized for the majority of the time. Fan regulation reduces oversizing while also contributing to energy savings and fan motor noise levels.

There are several options for controlling fan speed:

Fans with electronically commutated (EC) motors. An integrated commutation device controls the speed of an EC-motor by adjusting the position of the permanent-magnet rotor. EC-motor commutation circuits receive input via pulse-width modulation or, more commonly, 0-10V or 4-20mA from an external temperature sensor and regulate the speed from 10% to 100%. EC fans save the most energy when compared to other solutions, but the fan drive is more expensive than fans with conventional AC motors.

On/off control. This simple and inexpensive regulation system works by sensing fluid outlet temperature and requesting the activation or deactivation of a specific number of fans based on effective capacity. On-off control provides little power savings and little noise reduction. Furthermore, fan on-off cycling reduces its operational lifespan.

Cut-phase speed control. This controller modulates the speed of all fans at the same time by lowering the power voltage. It saves a lot of power and reduces noise, but it has a drawback: magnetic noise.

Inverter speed control also efficiently regulates all fans simultaneously by changing frequency. While providing significant power savings and noise reduction, the system is sophisticated and the most expensive of all.

A transformer-based speed controller provides very efficient regulation while producing no magnetic noise. Although continuous low-voltage operation mode saves energy and reduces noise, motor service life can be reduced.

Due to its low cost/quality ratio, cut-phase speed control is the most popular solution in modern systems based on AC-motor fans. Although EC-driven fans have grown in popularity, their use in low-temperature environments is limited.

Sound Emission

The amount of noise produced by a fan is directly proportional to its rotational speed: the faster the fan, the more noise it produces. Choosing fans with larger impellers capable of handling large air volumes results in less noise at the same airflow. At partial loads, such as at night, EC-motor fans with regulated speeds produce less noise.

Aside from that, fan noise can be reduced by using diffusers, which also improve system efficiency. Diffusers that decelerate the flow and convert a large proportion of the dynamic kinetic energy into static pressure save up to 25% energy while allowing an operating noise level of up to 7 dB(A) lower with the same airflow.

Heat Exchangers

Large industrial dry coolers typically use finned pack coils made of copper (tubes) and aluminium (fins), but smaller units can also be designed with all-aluminum microchannel cooling coils.

Coils must be cleaned on a regular basis in order to maintain heat transfer performance. Cleaning is a delicate procedure because fins can be easily damaged by excessive pressure, and fin thickness should be considered when selecting a dry cooler.

Tubeside pressure resistance of heat exchangers influences pump size and power draw; therefore, excessive pressure drops should be avoided whenever possible.

Corrosion Protection

Corrosion is one of the most common causes of failure in heat exchangers. The main reasons for this are a lack of anti-corrosive treatment/coating and insufficient cleaning.

Nowadays, a wide range of protective coatings are available, ranging from painting to the trivalent chromium process to electrophoretic epoxy coating, with varying levels of protection and cost.

Anti-corrosion treatments for cooling coils should be considered, especially for coastal installations and installations in industrial/urban zones with high levels of air pollution. Anti-corrosive treatment is also recommended for adiabatic-assisted systems. Coatings should be chosen based on the pollutants and chemicals found in the air.

Freeze Protection

In low ambient temperatures, make sure the process fluid contains enough antifreeze to keep heat exchangers and pipework from freezing. Antifreeze must be chosen with consideration for its viscosity, toxicity, and other properties. The most common antifreeze fluids are glycols, such as ethylene or propylene glycol. Even if the process fluid temperature is high enough, antifreeze fluids must be added in the event of a failure, maintenance, or downtime caused by other factors.

Adiabatic-Assisted Cooling Water Spray Systems

The water spray system uses intermittent atomized water spray on the inlet air stream to provide an adiabatic cooling effect, lowering air temperature closer to wet-bulb temperature, contributing to significant energy savings and increased duty. The spray system necessitates water treatment; sprayed water cannot be collected for reuse, making this type of adiabatic pre-cooling water-intensive.

Evaporative Pre-Cooling

The same is true for evaporative pre-cooling, in which the incoming air stream passes through pads made of cellulose or other materials, such as aluminium foil, that have been wetted with (tap) water. Water usually does not need to be extensively treated and can be collected for reuse. Overall, evaporatively cooled units save energy on fans and have higher heat rejection than conventional dry coolers.

Average local climate characteristics such as temperature and relative humidity should be considered when evaluating the seasonal and total efficiency of the methods described above.

Maintenance

To reduce operating costs and improve system reliability, it is necessary to evaluate the serviceability of dry coolers for specific installations of various designs: flatbed or V-shaped, with horizontal or vertical airflow.

Dry coolers require little maintenance. Typical dry cooler maintenance tasks include:

General inspection: identifying mechanical issues and damages, detecting excessive fan vibrations, and visually inspecting for traces of corrosion.

Cleaning of cooling coils. General debris such as leaves, paper, dust, and pollen can be removed with a brush and compressed air blown in the opposite direction of the airflow. Heavier fouling must be removed with a pressure water/steam jet washer against the airflow direction, using a neutral cleaning agent if necessary. Any cleaning fluids used should be safe for both tube and fin materials and coatings. It should be noted that improper cleaning agent use may be corrosive to heat exchanger materials.

Check for fluid leaks in heat exchangers and connections.

Check the electrical connections for tightness and the fan Amps consumption.

Examine all control devices and probes.

HVAC professionals should perform careful selection of the most appropriate dry cooler and associated options based on objective evaluation of the characteristics and requirements of the application.

Technicians should validate software-assisted selection to ensure that dry cooler specifications are tailored to all details of the application, cost-effective, and future-proof.

P.S. If you're looking for a fluid cooling solution, we'd love to hear about it and recommend the most efficient and cost-effective dry coolers. Please contact us at info@cstheatexchanger.com.