Dry Cooler Selection: What Do You Need to Know?

What do you need to know about choosing a dry cooler?

Dry coolers are heat rejection devices that are used to chill process liquids such as water, glycols, and oils by forcing heat energy into the atmosphere. Dry coolers, as opposed to wet cooling towers, are closed-circuit devices 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 a variety of other applications.

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

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 primary component parts: air-to-liquid heat exchangers and fan blowers. In most cases, axial fans are utilised, and heat exchangers are finned pack coils or, in small or medium systems, microchannel coils.

Dry cooler selection appears simple, and it is – until current client requirements such as optimised power consumption, reduced sound emission, or size constraints are carefully considered. The work becomes much more difficult given the abundance of technical solutions for fans, heat exchangers, and aiding components such as adiabatic pre-cooling. So, what factors should be considered while purchasing a dry cooler for a contemporary and effective cooling plant?

Baselines

Fluid inlet temperature, outlet fluid temperature (or flow rate), kind of fluid to cool, and ambient air temperature are the basic inputs for the selection process. A variety of other criteria and conditions, however, should be considered:

Collocation of units. Closely spaced units will almost certainly perform worse. Drawing of hot exhausted air by close units should be prevented by providing appropriate distances between units or positioning them on a sufficiently high gantry or frame. In the latter situation, small spaces between dry coolers must be filled with blind panels.

Wind speed and direction are dominant. Because severe winds can degrade performance, the installation site may need to be fenced, which may also isolate unit-borne noise.

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

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

Working fluid must contain antifreeze in sufficient concentration to prevent freezing of heat exchangers and pipes when operating in low ambient temperatures. AC-driven fans are suitable for use at very low ambient temperatures.

Noise from the fan. Selecting EC-driven fans to reduce noise output, for example, at night, by restricting fan rotation speed, is a recommended practise. Another efficient way of noise reduction for the entire cooling plant is the soundproof fence. Another efficient way of noise reduction for the entire cooling plant is the soundproof fence.

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

Fan speed controls and controlling signals of various types. Units may contain separate fluid temperature sensors and fan speed controllers, or they may be incorporated in higher-level systems that receive regulating signals from networked control devices.

Dry cooler controls may need to be added to the upper-level BMS system. As a result, when selecting control settings, the type of network protocol should be considered.

Drop in available water side pressure. High pressure decrease may increase pump size and power draw.

The electrical/control panel is generally mounted directly on the unit; but, in some circumstances, such as installations in cold areas, an electrical enclosure must be located remotely, indoors, or using a stand-alone weatherproof construction.

Individual switch-offs on fan blowers allow for hot switching or maintenance without requiring the machine to be shut down.

Kind of fluid connection.

Vibration dampers of many types.

Fluids in Process

Water, water-glycol combinations, and oils/special fluids are examples of common process liquids. Several inhibitors should be employed to prevent heat exchanger corrosion concerns in specific instances, such as when fluid may contain contaminants or its pH level is outside of the permissible (safe) range. In the former case, a strainer must be installed. Certain fluids may necessitate the use of special protective heat exchanger inner coatings.

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

To avoid oxidation, cavitation, and airlocks, external pipework must incorporate air vents.

Controlling the Fan Speed

Fan speed controllers alter motor speed in response to detected fluid temperature to maintain the fluid outlet temperature setpoint for dry coolers under any load. Fan control is critical since dry coolers are intended for maximum conditions - maximum ambient temperature and thermal load. In actuality, maximum load combined with maximum ambient temperature occurs only for a small portion of the year, making dry coolers enormous for the majority of the time. Fan regulation helps to reduce oversizing while also contributing to energy savings and fan motor noise levels.

There are several options for regulating fan speed:

Electronically commutated (EC) motors are used in fans. An integrated commutation device adjusts the position of the permanent-magnet rotor to control the speed of an EC-motor. EC-motor commutation circuits accept pulse-width modulation or, more commonly, 0-10V or 4-20mA from an external temperature sensor and adjust the speed from 10% to 100%. When compared to other solutions, EC fans give the greatest energy savings; nevertheless, the cost of the fan drive is higher than for fans equipped with ordinary AC motors.

On/off switch. This simple and inexpensive regulation system works by monitoring fluid outlet temperature and demanding that a particular number of fans be turned on or off based on effective capacity. On-off control saves little power and reduces noise just somewhat. Also, fan on-off cycling reduces its working lifespan.

Controlling the speed of the cut-phase. By lowering the power voltage, this controller adjusts the speed of all fans at the same time. It ensures significant power savings and noise reduction, but has a drawback in magnetic noise.

Inverter speed control also manages all fans simultaneously by adjusting frequency. While providing significant energy savings and noise reduction, the system is complicated and the most expensive of all.

A transformer-based speed controller enables extremely efficient regulation while producing no magnetic noise. Although continuous low-voltage operation mode provides significant energy savings and noise reduction, motor service life may be compromised.

Because of its low cost/quality ratio, cut-phase speed control is the most preferred method in modern systems based on AC-motor fans. Although EC-driven fans have grown in popularity, their usage in low-ambient settings is limited.

Emission of Sound

The amount of noise produced by a fan is proportional to its rotational speed: the faster the fan, the more noise it produces. Using fans with larger impellers capable of handling huge air volumes reduces noise while maintaining the same airflow. During partial loads, such as at night, EC-motor fans with regulated speed emit less noise.

Aside from that, fan noise can be decreased by using diffusers, which also boost system efficiency. Diffusers that slow the flow and convert a considerable part of the dynamic kinetic energy into static pressure save up to 25% energy while allowing for a 7 dB(A) lower operational noise level with the same airflow.

Exchangers of heat

Finned pack coils composed of copper (tubes) and aluminium (fins) are commonly used in large industrial dry coolers, although smaller units can also be constructed with all-aluminum microchannel cooling coils.

To maintain heat transfer performance, coils must be cleaned on a regular basis. Cleaning is a delicate technique because high pressure can quickly damage fins, and fin thickness should be considered while selecting a dry cooler.

Heat exchanger tubeside pressure resistance effects pump size and power draw; hence, excessive pressure decreases should be avoided wherever possible.

Corrosion Defense

Corrosion is one of the most serious problems with heat exchangers and the leading cause of failure. The primary causes of this are a lack of anti-corrosive treatment/coating and insufficient cleaning.

Currently, there is a wide range of protective coatings 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 areas with significant levels of air pollution. For adiabatic-assisted systems, anti-corrosive treatment is also recommended. Coatings should be chosen based on the reported contaminants and substances in the air.

Protection Against Freezing

In cold temperatures, make sure the process fluid contains enough antifreeze to keep heat exchangers and piping from freezing. Antifreeze must be chosen with consideration for viscosity, toxicity, and other qualities. Glycols, such as ethylene or propylene glycol, are the most often used antifreeze fluids. Even if the temperature of the process fluid is high enough, antifreeze fluids must be supplied in the event of a breakdown, maintenance, or downtime caused by other circumstances.

Spray Devices for Adiabatic-Assisted Cooling Water

The water spray system uses intermittent atomized water spray on the inlet air stream to create an adiabatic cooling effect, bringing air temperature closer to wet-bulb temperature and contributing to significant energy savings and higher duty. Water treatment is required for the spray system, and sprayed water cannot be collected for reuse, making this type of adiabatic pre-cooling water-intensive.

Pre-Cooling via Evaporation

The same is true with 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 does not usually need to be significantly treated and can be collected for reuse. Overall, evaporatively cooled devices save significant energy on fans and have higher heat rejection than standard dry coolers.

To evaluate the seasonal and total efficiency of the following methods, average local climate variables such as temperature and relative humidity should be considered.

Maintenance

To reduce operating costs and increase system dependability, it is required to assess the suitability of dry coolers for specific installations of various designs: flatbed or V-shaped, with horizontal or vertical airflow.

Dry coolers are relatively simple to maintain. Common dry cooler maintenance tasks include the following:

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

Cooling coil cleaning. Debris such as leaves, paper, dust, and pollen can be cleaned with a brush and compressed air blown in the opposite direction of the airflow. Heavier fouling must be removed using a pressure water/steam jet washer against the airflow direction and, if necessary, a neutral cleaning chemical. Any cleaning fluids used should be safe for both tube and fin materials as well as coatings. It should be noted that improper cleaning chemical use might be detrimental to heat exchanger materials.

Examine heat exchangers and connectors for fluid leaks.

Examine the electrical connections for tightness and the fan Amps usage.

Examine the control devices and probes.

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

Technicians should validate software-assisted selection to ensure that the dry cooler specification is suited to the details of the application, cost-effective, and future-proof.

P.S. If you're searching for a fluid cooling solution, let us know and we'll find the most efficient and cost-effective dry coolers for you. Please contact our staff at info@cstheatexchanger.com.