Microchannel heat exchangers improve the performance of chillers

In a drive to improve the energy efficiency of cooling products, microchannel heat exchangers (MCHE) have made an impact on the market. As an evolution of fin and tube heat exchangers, MCHE’s provide both efficiency and cost savings, along with high corrosion resistance, specifically in the air-cooled chiller market. 

What is a heat exchanger?

Heat exchangers enable the transfer of heat between two different mediums, without mixing or being in direct contact. They play a vital role in air-cooled chiller systems, where a condenser coil (heat exchanger) enables the transfer of unwanted heat from refrigerant to the air.

How do heat exchangers work?

In a traditional fin and tube heat exchanger, refrigerant will flow through narrow copper tubes (known as coils) on the inside of the heat exchanger and air flows through aluminium fins on the outside. 

In air-cooled chiller systems, the refrigerant in the thin tubes is carrying heat. In a step-by-step process, this heat is exchanged with the air. Firstly, the heated refrigerant in the tubes transfers it’s heat to the tube wall. A series of aluminium fins are connected to the outside of the tube wall. The heat then conducts through the tube wall to these fins. The fins play a crucial role as they increase the surface area available for air to flow over, allowing for the enhanced transfer of heat from the fins to the air. The heat moves from the fins to the passing air, to be released into the outside atmosphere.

How do microchannel heat exchangers work?

MCHE’s have taken the traditional fin and tube heat exchanger and developed it for the next generation. The operating principles are exactly the same, with refrigerant flowing through narrow tubes inside of the heat exchanger, transferring unwanted heat via fins to the air.

A microchannel unit will include two headers - an inlet header and an outlet header - that run either side of the heat exchanger. Between each header are a series of flat tubes, which in turn are divided into micro channels (smaller tubes within the flat tube, running its entire length). This increases the surface area and the heat exchanged via the tube wall. Louvered fins run in-between the tubes, to increase the surface area available to the air. The system is brazed together with non-corrosive fluxes and made of aluminium alloys. Refrigerant flows through the flat microchannel multiport tubes and air flows through the gaps in the fins. 

The clever thing about this system is the way it increases the surface area available to air and prolongs the time the refrigerant stays within the heat exchanger. The refrigerant enters through one of the headers and then passes through the micro channels in the flat tubes until it reaches the other header. Baffels within the header, control the direction the refrigerant flows. This forces the refrigerant through the tubes a number of times. The longer the refrigerant stays in the tube, the larger the transfer of thermal energy to the tube wall. 

As in traditional fin and tube heat exchangers, heat is conducted through the tube wall to the fins. Heat moves from the fins into the air and is released into the outside atmosphere.

Do microchannel heat exchangers make chiller systems more energy efficient?

The short answer is yes. The use of MCHE’s brings improved heat transfer and thermal performance. This is because:

• MCHE’s can use up to 50% less refrigerant 
• The manufacturing method of brazed assembly eliminates contact resistance for higher efficiency
• Flat microchannel multiport tubes have a smaller diameter than copper tubes, which enables higher heat exchange in single-phase heat transfer
• Flat microchannel multiport tubes have increased surface tension compared to copper tubes, causing stratified flow. This also enables higher heat exchange in two-phase heat transfer
• Microchannel units use louvered fins. This ensures a heat transfer coefficient 50% higher than with the rectangular fins usually found in fin and tube heat exchangers
• Louvered fins can reduce airflow turbulence and minimize air shadows
• Louvered fins produce lower airside pressure drop, which reduces fan power consumption
• MCHE’s have superior corrosion resistance, which increases lifespan. 

Manufacturers of MCHE’s are in agreement of their superior ability to transfer heat and importance within high efficiency HVAC systems. Microchannel heat exchangers offer a high level of reliability and durability, reduced refrigerant charge, decreased size and weight, along with enviable carbon footprint savings.

Microchannel vs fin and tube

Microchannel heat exchanger advantages:

• Energy efficient – some manufactures claim between 20-40% more efficient than fin and tube heat exchangers
• MCHE’s are much smaller and can weigh up to 60% less per heat exchanger, or up to 7% less per chiller
• Constructed robustly with a longer lifespan
• Easy to clean which improves serviceability
• Likely to cost less in materials
• Can improve the energy efficiencies of older systems when replacing fin and tube heat exchangers
• Quieter in operation, as smaller fans can be used
• More flexible in regard to layout and design.

Microchannel heat exchanger disadvantages:

• Condensate can be an issue and can block airflow over the fins 
• Because the unit is more complex, design features within the heat exchanger need to ensure correct refrigerant flow.  Poorly designed or manufactured units will not deliver desired results.

One of the big benefits of MCHE is the natural corrosive resistance offered by aluminium build, which can be further enhanced by protective coatings. The Carrier - Heat Exchanger Corrosion Durability Study by Advanced Materials Engineering, compared nine different coils including uncoated and coated MCHE, coated and uncoated copper and aluminium fin and tube systems, along with bare coils in both aluminium and copper. 

Unsurprisingly, a premium coated Micro Channel Heat Exchanger came top of all the tests. However, the uncoated MCHE still out performed traditional fin and tube heat exchangers that were uncoated, had precoated aluminum fins and used blygold coated coils with copper tubes and aluminium fins. The uncoated MCHE also out performed the bare coil with copper tubes and copper fins.

Certification for Chillers using microchannel heat exchangers

When choosing a chiller using MCHE, you want to ensure you select the right product for your application. Not only do you need to know its capabilities, but it’s vital you have information on its technical performance. This will help you to avoid any potential performance issues. 

Certification enables you to compare products objectively to make an informed decision. Benefits of choosing a certified product include:

• The product performance is evaluated according to the same criteria, and the results are expressed in the same unit of measurement, regardless of the country where the products are manufactured or marketed
• A certified product has its performance verified by an impartial, independent and competent accredited body
• Certified products comply with standards
• A product whose performance is certified will work according to the specifications stated by the manufacturer.

So if you want to view and compare chiller technology, our certified product directory is a good place to start. The easy to use search facility lets you view products by family, type, brand, model name and certificate number. 

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