Exploring high-efficiency commercial air conditioners with MCHE evaporators

Microchannel heat exchanger (MCHE) evaporators prove to be more efficient and cost-effective than traditional fin-and-tube evaporators.

Mustafa Yanik, Danfoss, Baltimore, Maryland
Exploring high-efficiency commercial air conditioners with MCHE evaporators

Figure 1: Basic design of MCHE: 1) baffle for multipass; 2) microchannel tube; 3) header; 4) end cap; 5) side plate for protection; 6) enhanced fins with louvers. All graphics courtesy: DanfossLearning objectives

Demand for commercial air conditioners continues to increase. In developing countries, economic growth coupled with a rising middle class creates a double-digit rise in the demand for commercial air conditioners. Air conditioning is now seen as a necessity, not a luxury, and is becoming commonplace in all nonresidential buildings. As a result of this ever-increasing use of air conditioning, on top of limited energy resources, the efficiency requirements for commercial air conditioners continue to expand in both developed and developing countries.

Microchannel heat exchanger technology

Figure 2: Examples of MCHE evaporators avoiding intermediate headers. (Left: Slab coil, Center: Two-row-folded, Right: A-coil-folded

Typically, attempting to increase the efficiency of traditional fin-and-tube heat exchanger technology would lead to the development of larger heat exchangers. This has two drawbacks. One is increased cost, which sometimes delays the decision to replace an older, less efficient system and counters the benefit of higher efficiency. The second is an increase in refrigerant charge, which raises the direct global warming potential (GWP) impact of the system.

Micro channel heat exchanger (MCHE) technology, on the other hand, has allowed HVAC original equipment manufacturer (OEMs) to increase the efficiency of heat exchangers without significant increases to cost or refrigerant use. As a result, MCHE condensers are widely used in air conditioning and refrigeration systems. MCHE evaporators also show similar benefits as condensers, but they bring unique challenges-such as 

Figure 3: Distributor in an MCHE evaporator.

refrigerant distribution and condensate management-that need to be considered in the heat exchanger and system design phases. As a result, the application of MCHE evaporators in commercial air conditioning systems is relatively new, with limited market penetration to date, but is increasing.

It's critical to realize how MCHE evaporators can increase the cost-effectiveness of high-efficiency commercial air conditioners. It also will discuss the challenges related to the application of MCHE evaporator technology and how they can be overcome.

Drivers and challenges of MCHE condensers and evaporators

MCHEs were developed for the automotive industry at the end of the 1980s and implemented as condensers in the early 1990s. This was followed by the evaporator. The development/implementation was driven by the high efficiency, compactness, and reliability provided by the MCHE technology. (See Figure 1).

MCHE technology has been recognized by the air conditioning and refrigeration industry since the early 2000s due to an increased focus on reducing refrigerant charge-a result of environmental regulations and cost. In addition, the cost of copper typically used in traditional fin-and-tube heat exchangers has been volatile and made it difficult to predict total heat exchanger cost.

To obtain the technology's maximum benefits, it's important to evaluate the physical differences of condensers and evaporators, as illustrated in Table 1.

Table 1: This table shows the direct drivers for MCHEs

Figure 4: MCHE evaporator 25 x 1,000 x 1,000 mm.

The total system charge depends on the entire refrigeration system. Although piping often remains with similar dimensions, buffers such as receivers and accumulators can typically be downsized. A compact refrigeration system using an MCHE condenser often achieves 30% to 40% less system charge. When an MCHE evaporator is included as well, a total system charge reduction of 45% to 60% can be reached. In addition to lowering the direct GWP impact of the air conditioning systems, the cost of refrigerant charge is significantly reduced.

The benefit of the size and weight reduction depends on the application, but is not neglegible when considering cost of transport, storage, and installation.

An all-aluminium design increases the resistance to galvanic corrosion, especially in MCHE condensers. By eliminating copper from the evaporator, the risk of corrosion caused by formicary also is eliminated.

By constructing MCHE heat exchangers from aluminum, a raw material that is widely available and has a relatively stable price, thereby eliminating the copper use, MCHEs are immune to copper price fluctuations 

that make the price of traditional fin-and-tube heat exchangers difficult to predict. The all-aluminium construction also makes MCHE easy to recycle, further reducing its environmental footprint. In many applications, the MCHE will offer reduced air-side pressure drop due to the reduced drag of flat tubes versus round, which means less noise and less power consumption of the fan.

Table 2: This table illustrates the challenges for MCHEs

Figure 5: Capacity as a function of refrigerant maldistribution at superheat conditions from 4° to13°F.

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