Discussion on copper-aluminum heat exchanger in light commercial air conditioning products.
by Chigo HVAC
1. Types of heat exchangers: The main types of heat exchangers are, among others, the conventional copper-aluminum heat exchanger, the aluminum-aluminum heat exchanger (aluminum tube) and the parallel flow heat exchanger (micro-pass channel).
1.1 Specifications of the conventional copper-aluminum heat exchanger:
1. φ9.52mm tube diameter, 21.65mm fin width;
2. φ7.94mm tube diameter, 19.05mm fin width;
3. φ7mm tube diameter, 13.37mm or 19.05mm fin width;
4. φ5mm diameter, 13.37mm fin width.
1.2 The aluminum-aluminum heat exchanger has an aluminum tube of φ9.52mm in diameter, it is commonly employed in the United States market. It presents two major problems, in terms of welding and corrosion, and its thermal efficiency is only 92% of that offered by the copper-aluminum heat exchanger.
1.3 The parallel tube heat exchanger carries out its process by means of a flat aluminium tube and aluminium fins. The efficiency of heat exchange is determined by the width of the heat exchanger. It offers an energy efficiency that is 1.2 to 1.4 times higher than that of the copper-aluminum heat exchanger. However, it is not as competitive when the price of copper goes down and a much larger investment is required.
2. Use of heat exchanger in light commercial air conditioning products:
2.1 Use of copper-aluminum heat exchanger in light commercial air conditioning products:
to. The unit air conditioner heat exchanger with a 7 mm diameter tube and 19.05 wide fins has been greatly improved and has better performance in heating and defrosting mode. The adoption of this standard will allow great savings in costs, the current standard diameter of the market is 7 mm. As part of Chigo's air conditioning solutions, this type of product is also offered.
b. Heat exchanger fin distance: 1.4 mm for outdoor units and 1.5-1.7 mm for indoor units. (The distances of the fins are related to the volume of air.)
c. Heat exchanger rows: 2-4 rows for indoor units and 1-2 rows for outdoor units.
2.2 The use of the aluminium-aluminium heat exchanger in light commercial air conditioning products:
to. The aluminum-aluminum heat exchanger is preferred in the Latin American region due to the dry climate. It is commonly used in the roof unit and in the unit outdoor unit.
b. The aluminium-aluminium heat exchanger should not be used in the heat pump or evaporator because these two parts usually remain submerged in water for a long time, which will accelerate corrosion.
2.3 Use of φ5mm copper-aluminum heat exchanger in light commercial air conditioning products:
to. The φ5mm copper-aluminium heat exchanger is used in units with low cooling capacity: it is usually used in indoor units with a capacity of less than 18,000 btu and in outdoor units with a capacity of less than 24,000 btu.
b. The main advantage is the ratio between the low cost of the material and the thermal efficiency of the unit/.
c. It is currently used in chigo residential air conditioning, for example in the wall unit, and is also used in commercial air conditioning, in the 1-2 HP duct unit.
3. Heat exchanger circuits
The design of the heat exchange refers to the function between the evaporator and the condenser.
This defines that the high temperature side is the condenser while the low temperature side is the evaporator.
3.1 The influence of the direction of circuit flow on heat exchange. The flow direction includes:
to. The relationship between heat exchangers and the downdraft and updraft of the wind field. Typically, the upstream design should be implemented in a multi-row heat exchanger. The downstream design should be adopted in indoor units with heat pump, taking into account the water outlet of the evaporator or some other possible problem that may occur in the heating mode.
b. The vertical and horizontal position of the inlet and outlet pipe. To get the most out of flotation and gravity, we must use a "vertical position for liquid inlet" and "vertical position for gas outlet" design.
3.2 Determine the number of circuits
to. Note the pressure loss, if there are more circuits in the evaporator and condenser that means there is only one shorter circuit in the heat exchanger and there is less pressure loss, as well as an improvement in the capacity of the system. However, this is not completely accurate for the following two reasons:
(1) When the evaporator and condenser are very large, having more circuits generates a low flow volume and a lower flow rate in each circuit, which causes the oil to accumulate on the inner surface of the pipe and form an oil film when the system has a very bad oil return. This will decrease the efficiency of heat exchange and have a very negative impact on heating capacity. When the system has insufficient heating capacity, consideration should be given to reducing the number of circuits.
(2) When there are more circuits this causes each circuit to be irregular, which produces incomplete evaporation and other problems. As a result, capacity decreases. Usually, the number of circuits has a marked incidence on the low temperature side and a slight effect on the high temperature side in the cooling mode. A pressure drop of 0.3Kg will not have much impact on the condensation temperature, since that is the cooling capacity of the system. Usually, in the cooling mode, the number of circuits has more obvious effects on the low temperature side and a less marked effect on the high temperature side, the reason is that a pressure drop of 0.3kg has little effect on the condensation temperature, therefore it has little effect on the cooling capacity.
But the increase in the discharge pressure of the compressor will cause an increase in the energy consumption of the compressor, therefore, to avoid problems caused by the power supply, you can try to optimize the number of circuits on the high temperature side.
b. For a given system there is a precise optimal number of circuits, such a number must be determined according to the size of the heat exchanger, the volume of the compressor discharge and the type of refrigerant.
c. The extension of the circuit is also a determining factor in establishing the number of circuits. Given the incidence of the type of refrigerant and the diameter of the pipe, the influence of the dynamic viscosity of the liquid and the diameter of the pipe on the pressure drop is the fundamental factor in establishing the extension of the circuit. If you have the following dynamic liquid viscosity: R410A=R22*70%, the extent of R410A must be greater than that of R22.
3.3 Design of the distribution of heat exchangers
The design of the distribution of high-efficiency heat exchangers is the key aspect in the proportion of heat exchange, apart from the considerations included in A and B, above, the design must also follow the following rules:
to. The design of the distribution should follow the principle of "liquid phase through a single channel, gas phase through several channels", that is, the principle of less input and more output of the evaporator design, which will improve the mass flow rate of the refrigerant in a dry zone. Compared with the method that consists of directly dividing into several flow channels, this distribution design does not divide any flow channel when the degree of dryness of the refrigerant is low, this will increase the mass flow rate of the refrigerant, and increase the pressure drop a little, therefore, the heat transfer coefficient in boiling in flow can be improved.
When the degree of dryness is high, a flow channel, or very small flow channels, will cause a very high velocity of the refrigerant in gas and a high pressure drop, therefore, in these conditions more circuits are needed to effectively decrease the pressure drop.
b. The design of the distribution must avoid the influence of overheating and ensure that the inlet and outlet pipe are not located adjacently on the same side, in addition it must follow the principle of the minimum temperature difference of adjacent pipes (that there is no overheating in adjacent rows).
c. The extent of each circuit of the distribution must be related to the distribution of the wind field. In a uniform wind field, the same extension is normally implemented in the circuits. But in a heterogeneous wind field the extension of the circuit in the part of high air volume must be reduced and the extension of the circuit in the part of the low air volume must be increased.
The circuit extension in the high air volume part should be reduced and the circuit extension in the low air volume part should be increased to ensure uniform heat exchange.
