Exploring high-efficiency commercial air conditioners with MCHE evaporators
- Define micro channel heat exchanger (MCHE) technology.
- Explore the benefits of MCHE technology.
- Analyze the applications and challenges of MCHE technology in commercial air conditioning systems.
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
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
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.
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.
The MCHE condenser has matured to the point where there are no significant challenges to overcome.
However, the MCHE evaporator is more challenging in many ways, and the design of a commercial air conditioning system with MCHE evaporators requires a collaboration between the OEM and the heat exchanger manufacturer. (See Figure 2).
Refrigerant distribution versus MCHE evaporator performance
In the traditional fin-and-tube system, a liquid distributor is used to ensure equal refrigerant quality in each circuit/tube of the evaporator. In an MCHE evaporator, the distributor is integrated into the header. Due to concerns about refrigerant maldistribution, multirow MCHE coils with intermediate headers would not perform well. As a result, a folded coil is required whenever a two-row design is needed for high performance. (See Figure 3).
The flow and spray patterns are very important for a uniform refrigerant distribution. A significant amount of research has been done to understand the basics and develop functional products. Important factors to consider include the flow pattern in the distributor tube, orifice size and spacing, spray angles, and pressure-drop equalization that can be improved by multiple inlet and outlet connections.
The sensitivity of the MCHE evaporator coil performance to refrigerant-flow maldistribution as well as phase maldistribution was simulated by dividing an MCHE evaporator into two sections and then prescribing different conditions/flow rates to each section:
MCHE evaporator design used in simulation: 25 x 1,000 x 1,000 mm – 1.1 mm fin pitch
Conditions: Air 80°F/50% RH – 500 ft/min R410A – Tliq 100°F – Tsat evap 51°F
(See Figure 4 and Figure 5).
Refrigerant-flow maldistribution has a significant impact on the capacity, but the impact is not very dependent on superheat. (See Figure 6)
Refrigerant-phase maldistribution has less impact as compared with mass flow-rate maldistribution, although the impact is, again, not very dependent on superheat.
Airflow distribution versus MCHE evaporator performance
A similar simulation was done, but in this case, the sections were divided as left-to-right as well as top-to-bottom to understand the sensitivity of coil performance to airflow-rate distribution. The refrigerant flow was kept uniform in the simulations. (See Figure 7).
Conditions: Air 80°F/50%RH – 500 ft/min R410A – Tliq 100°F – Tsat evap 51°F
(See Figure 8).
The maldistribution of air left-to-right air has a severe impact on the capacity and cannot be neglected in the design of an application. The top-to-bottom maldistribution has a relative minor impact.
Figure 9 shows a significant difference between an MCHE evaporator and a traditional fin-and-tube evaporator when it comes to the design of the airflow distribution through the coil.
Two-Row MCHE in reversible systems
In reversible systems, sometimes a two-row MCHE coil is needed to meet the capacity requirement of the indoor coil. In those instances,the two-row MCHE should be constructed from a large MCHE folded along the middle, to avoid intermediate headers that could cause refrigerant maldistribution. (See Figure 10).
The two-row MCHE should be connected so that it works in counter flow in heating mode (condensing) and in parallel flow/concurrent flow in cooling mode (evaporating). This arrangement could result in up to a 10% increase in capacity in heating mode, with a limited capacity drop of about 4% or less in cooling mode based on tests compared with opposite airflow direction. (See Figure 11).
MCHE evaporators for multicircuit systems
When a multicircuit system is needed, the face-split configuration with the installation of MCHE evaporators placed side by side is preferred as over a top-to-bottom configuration, due to easier and better distribution of the refrigerant as well as smaller headers (lower cost) and higher refrigerant velocities (higher performance). In addition, each evaporator is exposed to equal air temperature/velocity conditions. (See Figure 12).
The row-split configuration, where the evaporators are placed behind each other, has thermodynamic benefits in terms of system efficiency, but can be challanging for some applications due to uneven thermodynamic conditions. If this configuration is chosen, it is preferable for the upstream coil to have the higher capacity due to the higher inlet-air temperature. (See Figure 13).
Comparison test of fin-and-tubes versus MCHE evaporators in a 20-kW commercial air conditioner
Based on tests conducted by Danfoss on a 20-kW commerical R-410A reversible heat pump, the MCHE evaporator offers a 66% weight reduction, 50% reduction in depth, and 18% reduction in refrigerant charge as compared with a fin-and-tube coil, while meeting the same performance requirements. (See Table 3).
Compared with the traditional fin-and-tube evaporator, the MCHE evaporator offers advantages of reduced refrigerant charge, size, and weight reduction. These advantages boost the cost-effectiveness of the commercial air conditioning system while keeping capacity and Energy Efficiency Ratio (EER) at the same level or better.
Mustafa Yanik is a global application expert and manager of heat exchangers and air conditioning at Danfoss.