I NEED A POWER POINT PRESENTATION ON MICRO HEAT EXCHANGERS

micro heat exchanger ppt

Abstract
Micro-channel heat exchanger(MCHX) has been increasingly applied in HVAC&R(Heating, Ventilation, and Air Conditioning & Refrigeration) field due to its higher efficiently heat transfer rate, more compact structure, lower cost. The characteristics of micro-channel heat transfer and fluid dynamics are summarized in this paper. The methods about optimizations (ie, geometry and thermodynamic performance) and the advantages and disadvantages of the MCHX are analyzed.

Design Characteristics
With an identical front surface on the airside, the micro-channel heat exchanger provides up to 10 percent added efficiency compared to a conventional fin and tube design. A lower thickness gives up to 50 percent lower pressure drop on the air side. This creates potential for lower energy consumption of the fan or, if desired, increased airflow. (Sensitivity to fouling can be expected to be lower.)

On the refrigerant side, the lower internal volume in the heat exchanger decreases the refrigerant charge.

Savings from index 100:
• 30 on required space
• 40 on heat exchanger weight
• 30 on the cost (visit the comparison table)

This compact design based on multi-port extrusions (MPEs) demands controlled atmosphere brazing (CAB), an oven-based process. Sapa supplies Micro-channel tubes and manifolds needed to manufacture brazed heat exchangers. The parts can be delivered with Hybraz coating, ready for assembly and oven brazing. Hybraz is a combined flux and filler material. For more information, visit the Hybraz page.

Increased Corrosion Resistance
Conventional coil after salt test

MPE coil after salt test
Conventional coil vs All-aluminium micro-channel heat exchanger after 5000h salt mist / amonium sulphate test
Unlike conventional coils, the micro-channel heat exchanger is made entirely of aluminium. This one-metal concept eliminates galvanic currents that are generated when different metals touch in conventional coils. Many comparative tests, including the salt mist and ammonium sulphate test, provide proof of the increased corrosion resistance of the all-aluminium micro-channel heat exchanger. Micro-channel heat exchangers offer three-and-a-half times higher corrosion resistance than traditional copper/ aluminium coils.



Environmentally friendly
Micro-channel heat exchangers have lower internal volume and a smaller refrigerant charge is needed. The smaller charges imply decreased use of refrigerants.



Condensation of water on evaporator fins.
Fin design A
Fin design 2
Fin design 3
Examples of fins designed for water drain-ability
Operation in subfreezing conditions: Very high fin density (small distance between fins) can be achieved with MPE heat exchanger design. The cold surface of the fin will normally create condensation of water. If the fin density is too high, then the condensed water cannot drain by gravity or airflow. The fin density shall also be balanced with the speed of frost formation and drainability under defrosting. sapa has performed wind tunnel tests of condensation, drainability, frost formation and defrosting. Based on its in-house knowledge, Sapa supports customers in brazed heat exchanger design. Fin design is the key.

Internal flow design
Many internal flow concepts can be realized with all-aluminium micro-channel heat exchangers. In a condenser design, it is easy to tailor the internal port cross section to the specific volume of the refrigerant. As shown above, there is a reduction of port section downstream alongside the condensation process in the condenser.

Operating Principles:

Micro-scale heat exchangers or micro structured heat exchangers are heat exchangers in which a fluid flows in a lateral direction in a confined area such as a tube or small cavity that dimensions are below the size of 1mm. Typically the fluid flows through a cavity which is called a mirochannel. This technology exploits enhanced heat transfer resulting from structurally constraining streams to flow in microchannels, which reduces resistance to transferring heat. Fluid flowing through the channels on a plate evaporates or condenses, and heat is transferred. Micro heat exchangers have been demonstrated with high convective heat transfer coefficients ranging form 10,000 to 35,000 watts/m2-°C, or about one order of magnitude higher than typically seen in conventional heat exchangers with very low pressure drops, typically 1 or 2 psi. The basic operating principle of these devices goes back to the convective heat transfer within the flows of the microchannels.

h=Nu(kd)

In this equation h is the heat transfer coefficient of the microscale heat exchanger, Nu is the Nusselt number which is about 3.65, k is the thermal conductivity of the working fluid and d is the diameter of the microchannel which the fluid flows through. From this equation one can tell see how the size of the channel directly affects the heat transfer coefficient of the heat exchanger, as the diameter is decreased the heat transfer coefficient increases.

Different Types of Microscale Heat Exchangers:

The different types of microscale heat exchangers are the same as the different classifications of conventional heat exchangers. They have either one or two passages for the fluid to flow through.

• One fluid:

When there is only one fluid and one passage in the heat exchanger the fluid is used to transfer the heat to another location. Application of this kind of heat exchangers is usually found in electronics to transfer heat into the fluid and out of the electronic device.

• Two Fluids:

When there are two fluids and two passages they are usually classified by the direction in which the fluids flow by each other. Microscale heat exchangers can either be cross flow or counter flow heat exchangers.

• Counter Flow

Counter flow micro scale heat exchangers work the same way as macro-scale counter flow heat exchangers. In a counter flow heat exchanger the two fluids flow in opposite directions of each other. The fluids enter the heat exchanger at opposite ends. The cooler fluids exits the counter flow microscale heat exchanger at the end where the hot fluid enters therefore the cooler fluid will approach the inlet temperature of the hot fluid. Counter flow microscale heat exchangers are more efficient than cross flow microscale heat exchangers.