Amid the global shift towards carbon neutrality, air-source heat pumps (ASHP) have emerged as efficient and environmentally beneficial to traditional boilers, attracting substantial attention in recent years. Contrary to the cooling process, where the condenser is inside, the heating process uses an indoor condenser and an outdoor evaporator in ASHPs.
Microchannel evaporators are being increasingly used in air conditioning and refrigeration systems, since they aid in reducing refrigerant charge, enhancing heat transfer performance, lowering tube-side pressure drop, and reducing system size. However, as with other types of evaporators, microchannel evaporators face the issue of refrigerant maldistribution. This phenomenon leads to unequal superheated vapor at tube exits and prevents optimum utilization of the heat exchanger surface.
Microchannel evaporators provide several advantages over traditional coils, the most important of which are high heat transfer rates and low internal volume, and have been widely studied with a focus on refrigerant distribution and condensate behavior. The present study reviews the impacts of refrigerant charge quantity and arrangement of refrigerant inlets and outlets on evaporator performance.
Retail chains in many countries are reviewing their carbon footprint and are focusing on refrigeration systems, particularly on choosing non-HFC, environment-friendly gases. While transiting to lower GWP refrigerants, among which flammable and mildly flammable gases like propane and HFOs, refrigerant quantity and, correspondingly, gas circuit volumes become essential factors in systems’ design.
High global warming potential refrigerants for refrigeration systems are being phased out because they are one of the culprits of climate change, while, at the same time, nearly 50 alternative refrigerants are classified as environment-safe.
Propane (R290) is a naturally occurring hydrocarbon used successfully in industrial refrigeration for many years received attention from designers, contractors, operators, and retailers to reduce greenhouse emissions. An indication of the increasing acceptance of propane is its adoption by many key customers from large supermarket and shopping mall chains to franchise food stores and minimarts at gas stations throughout the world. The latest trend in this market segment is adopting microchannel heat exchangers – condensers and evaporators.
Microchannel coils have become a mainstay in the HVAC industry for most air-cooled chillers and other air conditioning equipment. Aluminum microchannel heat exchangers (often abbreviated as MCHE or MCHX) offer a line of advantages for both manufacturers and end-users:
- Reduced refrigerant footprint: internal volume of a microchannel condenser is about up to 60% smaller compared to a finned pack coil with copper tubes of the same capacity.
- High corrosion resistance contributes to longer coil service life.
- Lower required fan power draw as air pressure resistance of MCHE coils is low.
- As a cheaper material, aluminum help keep condenser costs down.
- Low weight.
Dry coolers are heat rejection devices used to cool down process liquids such as water, glycols, oils by forced dissipation of heat energy to the atmosphere. In contrast to wet cooling towers, dry coolers are closed-circuit systems, involving no direct contact between air and process fluid.
Dry coolers can be used as stand-alone fluid coolers or as a part of an air conditioning or refrigeration system to cool fluids in intermediate heat exchangers, water chillers’ evaporators, and for many other purposes.
Customers are increasingly moving towards air conditioning and refrigeration equipment running on lower-GWP alternatives than currently widely used refrigerant R410a.
The current restrictions, such as Europe’s F-Gas regulations, have started the phase-down of HFCs, including R410a, to slow global warming and reduce direct carbon emissions. With the start of HFCs phase-down, prices and availability of these refrigerants were significantly affected and led to a growing demand for lower-GWP alternatives.
Free Cooling Concept
Free cooling is a method of cooling by using ambient air rather than performing the refrigeration cycle with, e.g., with a chiller. Many cooling systems engaged for air conditioning or industrial cooling require chilled water throughout the year. During cold seasons, ambient air is cool enough, colder than the supply chilled water setpoint, and mechanical cooling with the chiller’s compressors can be replaced with the free cooling operation. Even in warm climates, free cooling can produce energy savings, as fan motors consume much less energy than compressors.
Capacity control is in high demand for many cooling systems where cooling loads are not constant, and adjusting the system’s capacity to meet the prevailing load is essential for optimum efficiency. Efficiency primarily depends on compressor suction and discharge pressures, with the influence of secondary factors like useful and non-useful superheat and subcooling. As the screw compressor unloads, its suction pressure lowers (at the fixed condensing temperature), and efficiency decreases. Capacity control mechanisms allow maintaining the desired suction pressure by adjusting the refrigerant flow through the compressor.
Microchannel coils are here to stay. Their use is rapidly spreading to a variety of HVAC applications. Microchannel heat exchangers are stronger, lighter, offer higher heat transfer compared to other types of air-to-refrigerant coils. They are smaller in size and also in internal volume, resulting in a refrigerant charge that is significantly reduced.
Brazed all-aluminum microchannel coils perform superior heat transfer efficiency and capacity at the compact size compared with the conventional tube-fin heat exchangers. Another important factor of growing of microchannel heat exchanger popularity is the high corrosion resistance of aluminum, which is, however, achievable through the careful selection of aluminum alloys, surface treatments, and other protective measures.
Every chiller or chiller plant should be, for the most part, relatively self-reliant. Yet like all heavy machinery, regular preventive maintenance can improve performance, prevent breakdowns, and extend the service life of the equipment. Chillers should be inspected leastwise two times a year if they are only used during the cooling seasons and four times a year in case of continuous operation.
We recommend the following procedures to keep your chiller system at peak performance and avoid unexpected shutdowns and costly downtimes. The below checklist has been compiled to help chiller plant operators conduct detailed and reliable maintenance on air-cooled and water-cooled chillers.
Air-conditioning and refrigeration industries have been evolving rapidly over the past decades. The latest energy efficiency regulations and phase-out of ozone-depleting refrigerants spur further development of air-to-refrigerant heat exchangers, making microchannel heat exchangers the better alternative to conventional heat exchangers for the majority of HVAC manufacturers. Heat transfer technology based on microchannel coils offers the largest potential for improving energy efficiency, reducing refrigerant volumes required, while also providing a number of other benefits.
Microchannel heat exchangers have been broadly used in automotive applications due to their advantages of compactness, low weight, and high efficiency. There are numerous heat exchangers designs based on microchannel technology with folded fins and flat tubes which are connected to manifolds. These designs are predominantly used for automotive radiators, condensers, and, more recently, automotive air conditioning evaporators.
Cooling towers are heat-transfer devices that are used to reject heat loads to the atmosphere. Cooling towers are used to cool circulating water in a wide variety of applications such as process cooling and HVAC systems.
Working Principle and Differences
A chiller is a heat transferring device that uses mechanical refrigeration to remove heat from a continuous flow of process liquid and transfers it to the environment, thus lowering the temperature of the process liquid. Read more
Corrosion, the deterioration of metals and alloys through a physical and/or chemical reaction with the environment, is expensive. Heat exchangers used in HVAC equipment, specifically condenser and cooling/heating coils are exposed in the environment, and this can lead to corrosion, failures, and performance degradation of the equipment in the cases of improper heat exchanger protection in corrosive locations.
Compact heat exchangers, especially those of serpentine design, are widely used for cooling in many industries, products and systems, from consumer products to IT cooling to military applications. In recent times, with the advances in microfluidics and progress in manufacturing, compact microchannel heat exchangers are becoming widespread. The principal idea behind using microchannel heat exchangers is their potential for enhanced heat transfer capabilities. The use of such heat exchangers enables the transfer of large amounts of heat while benefiting from high heat transfer coefficients.
Single-phase heat exchangers are widely used today for a variety of cooling and heating applications and in different types of equipment. These include free cooling systems, dry coolers, air conditioners, and more, where the most common media is water or glycol mixtures. Microchannel coils in practice became the norm in condenser applications, and, more recently, microchannel heat exchangers became an alternative to finned tube coils that used traditionally for water applications.
Amount of electrical power consumed by cooling systems in energy-intensive industries like data centers and manufacturing plants is huge, especially in hot climate locations, and this fuels the development of new, energy-efficient cooling solutions and equipment for mission-critical applications. One of the more promising solutions is evaporative cooling combined with the conventional compressor-based system.
From an energy usage point of view, chillers are the most energy-intensive equipment to run and the key components within a chiller that impact performance and energy consumption are compressors. Turbocor compressors with built-in variable speed control can offer substantial energy savings of 50-60% compared to chillers equipped with fixed-speed compressors. In addition to the stepless capacity control, the Turbocor compressors employ built-in inlet guide vanes to trim the compressor capacity at low loads.
Closed-loop process cooling systems are gaining attention among a growing number of industries with the introduction of more stringent regulations on water use and increasing costs for water purchase and discharge. A hybrid cooling system where the dry cooling and evaporative cooling technologies are taken together is an attractive alternative to open-loop cooling towers in terms of both water saving and efficiency.
For over three decades now, air-to-refrigerant microchannel heat exchangers have been successfully used in a number of industries, including automotive and chemical processing. More than a decade ago, microchannel heat exchangers were introduced in the HVAC industry; but it must be noted that requirements concerning heat exchangers and their operating conditions in refrigeration and air conditioning systems are different from those applicable in the automotive and other industries. Because of this, the first adaptations of microchannel heat exchangers to HVAC applications, principally as condensers, were not entirely successful.
In recent times, microchannel heat exchangers (MCHE) have started to attract attention from the HVAC industry as an alternative to fin/tube evaporators. The enhancement of heat transfer and compactness are major advantages of microchannel heat exchangers, as well as reduced refrigerant charge, lower airside pressure drop, and a reduction in materials used for manufacturing. Under current tight regulations of energy efficiency, microchannel heat exchangers are appreciated by many leading manufacturers.
In most developed countries, a large amount of energy use of about 6% of the total is for air conditioning and refrigeration. The same is true for many hot countries where air conditioning systems use up to 40% of energy in the cities, and this could be a major problem as developing economies continue to grow.
For decades, data centers have used raised floor systems to deliver cooled air to the server racks. Cooled air discharged by a computer room air conditioner (CRAC – a split air conditioning system or system with fluid coolers) or air handler (CRAH – a chilled water cooling system connected to air-cooled or water-cooled chillers) creates high pressure in the underfloor space. Read more
Corrosion is the deterioration of metals arising from the chemical reactions between a metal and the surrounding environment. In HVAC equipment, such as air-cooled chillers, condensers, and dry coolers, corrosion of the heat exchangers may lead to performance and efficiency losses, as well as to equipment failures.
In order to avoid unwanted corrosion, microchannel heat exchangers and chilled water system, in general, must be properly protected based on environmental conditions.
Adiabatic cooling was first used in the Roman Empire. For 2000 years, people have been using water evaporation to cool down the surrounding air in all types of climate systems, including modern, highly advanced cooling systems, where engineers combine this ancient principle with modern technologies.
When it comes to chilled water plant optimization, the control strategy based on management of individual water chillers is not an optimum approach to achieve the best results in terms of energy efficiency.
Individual control of the water chillers with common pipework will cause unnecessary cycling of compressors and associated excessive power consumption as a response to changes in cooling demand; consequently, compressor cycling will lead to unstable return water temperature and increased wear of compressors.
Air-based free cooling uses low-temperature ambient air as a cooling medium for chilled water systems, thus providing significant opportunities to generate energy savings. The more time the chilled water system can operate in the free cooling mode, the higher the energy savings it can deliver.
The HVAC and process industries are making a transition to microchannel heat exchangers. This is evidenced by the extensive use of microchannel condensers by major HVAC manufacturers and many smaller OEMs manufacturing cooling and refrigeration equipment. Significant energy efficiency improvements drive the trend, the need for smaller equipment footprints and weight, conversion to lower GWP refrigerants, and the ongoing rise in the cost of copper.
Microchannel coils are becoming quite popular as an alternative to bulky finned tube heat exchangers. For the desired capacity, microchannel coils are smaller in size resulting in less refrigerant charge in the system, offer higher heat transfer performance, and lower weight. However, until now, microchannel coils used only to a very limited extent in HVAC units exposed to freezing conditions like heat pumps.
by Steven Wand
Although not traditionally used in industrial process applications, microchannel designs may offer advantages over traditional coil technologies in most process cooling applications. Long considered a coil technology primarily used in commercial and residential air-conditioning, microchannel coils are being used by more manufacturers who have discovered how they can improve process cooling applications.
by Lance Robinson
I vote for fix. Anytime you can fix a condenser as opposed to replacing it, is a good day for all in the repair business. Less time = more money. Customers are happier and all around better vibe. Replacement of perfectly good coil makes no sense. Instead make the fix in less than 5 minutes with a propane torch and a few simple instruments all HVAC techs have on hand.
Approximately 10% of heat exchanger failures are connected with leaks caused by formicary corrosion. Organic compounds remain on copper tubes during manufacturing and fabrication of the HVAC equipment, are able to advance to formicary corrosion only with the simultaneous presence of moisture, oxygen, and acids. Read more
by Carter Stanfield
Aluminum coils are here to stay. Not only have they been applied successfully for years by Kaltra in their spiney fin condenser coils, they have also been used for years as evaporator coils in refrigerators.
Since the introduction of the next generation all-aluminum microchannel coils, their acceptance in the process and HVAC industries has been nothing short of remarkable! The microchannel coil advantages are the future and rapidly becoming the norm because of the many benefits they provide in new HVAC equipment.
All-aluminium microchannel heat exchanging products are now being widely used in the HVAC industry: they provide higher performance, smaller size, features and capabilities not found in traditional fin/tube heat exchangers.
WHO WE ARE
Kaltra is a customer-focused developer and supplier of reliable, energy efficient thermal management equipment and solutions.
From the professionalism of our research and development engineers to the wide experience of our service team, we are dedicated to delivering you the best service possible.