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MicroHX (Microscale Heat Exchangers)
Microscale heat exchangers or micro heat exchangers (microHX) are heat exchangers in which one or more fluids flow in a confined area (tube, channel, …) that has dimensions on the order of 100 micrometers (1 µm to 1000 µm). The confined area based on its shape is called: microchannel, microtube, microcavity, etc. Small sizes of the channels reduce the resistance to heat transfer. That and very high surface to volume ratios, cause very high rates of heat transfer in these devices. The convective heat transfer coefficients in microchannels can be an order of magnitude higher that conventional channels.
Similar to conventional heat exchangers, several flow configurations may be seen in different microHX’s like one-fluid or two-fluid heat exchangers, counter-flow or parallel or cross flow configurations, etc. A very familiar example of one-fluid microHX’s can be found in these days computers where a microHX is used for cooling down electronic parts (like CPUs).
Some other applications are: microchannel fuel vaporizer (fuel cell), Commercial and residential heating/cooling, electronics cooling, biomedicine.
One obvious advantage of a microHX is its size! Although the channel dimensions are on the orders of microns (micrometers), the size of a microHX can be on the orders or centimeters or even meters. To reach the power of a conventional heat exchanger, hundreds of microscale channels should be put together. For example a conventional heat exchangers transfers 100 Joules of heat per second while a microscale heat exchanger can transfer (for example) 0.5 Joules per second, therefore 200 of this microHX is needed to transfer the same amount of heat per second. The major point here is that the total size of 200 microHX’s is smaller than the conventional heat exchanger.
As the size of the device goes down, the weight of the device goes down too. Less material will be used in microHX’s.
One other way of reducing the weight of the system is using light weighted materials, for example using aluminum instead of stainless steel. The problem is aluminum might not be able to tolerate the temperatures; however, higher heat transfer rates and innovative designs can reduce the operating temperatures to levels tolerable with lighter materials. Even plastic is used to make microHX’s these days.
Heat transfers through surfaces and the more is the surface area, the better is heat transfer. Microchannels (or microtubes, etc.) have high surface to volume ratios and as the dimension decreases, this ratio increases. That causes less resistance for heat transfer. It can be clearly seen from the convective heat transfer equation: h = (Nu×k)/D
where k is the fluid thermal conductivity, D is the dimension and Nu is the Nusselt number. Nusselt number is a constant value for laminar flow in a channel (i.e. 3.65) and also thermal conductivity of a fluid is constant in this case. Therefore the smaller is the dimension; the larger is the heat transfer coefficient.
Since less material in being used for fabrication of a microHX, the cost would be less. However, the fabrication technique or tool can be very expensive compared to a conventional heat exchanger. These tool costs can be significantly reduced with an appropriate manufacturing plan and mass production.
Besides all these advantages, microHXs have some down points too. The major problems are:
Because of their small hydraulic diameters, microscale heat exchangers can have large pressure drops. Therefore there need to be a balance between the desired heat transfer rates and pressure drops. One way to escape from high pressure drops is to have microchannels parallel to each other.
High surface area to volume ratios increases heat loss and can be significant in cases where the operating temperature in the microHX is very different from ambient temperature. Designing the microHX arrangement in such a way that the final shape of the device is close to a cube or sphere helps to reduce heat losses.
Sometimes headering of a microHX can be a problem and may get larger than the heat exchanger. There are several innovative header designs available that can reduce the size and weight of the headers. The following picture shows headers size compared to the size of the microHX.
