Heat exchangers, as the name implies, are devices in which two fluids at different temperatures exchange energy in the form of heat, without production or consumption of mechanical or electrical energy from the outside.
Fluid refers to any substance in the liquid or gas state.
Heat is transferred by convection, between the fluids and the corresponding solid surfaces these come into contact with, and by conduction through the wall that separates the two fluids.
Heat exchangers must be designed so as to extend contact between the two fluids, maximising the amount of energy exchanged. Consequently, the materials used feature high thermal conductivity, such as copper, aluminium or steel, and construction is aimed at expanding the heat exchange surfaces between the two fluids as much as possible.
In refrigeration circuits, heat exchangers generally act as condensers or evaporators for the refrigerant, at times performing both functions alternately.
The three most commonly-used types of heat exchanger in the HVAC/R systems are finned coils, tube bundles (including flooded) and plate heat exchangers.
More rarely and typically only for heat exchange not involving condensation or evaporation (for example, heat recovery), tube-in-tube heat exchangers are also used.
The type of heat exchanger essentially depends on the fluid that the refrigerant needs to exchange heat with; finned coils are used with air, the other types are used with water.
Finned coil heat exchangers
These are made up of a series of tubes carrying the refrigerant, and a compact group of fins, placed perpendicular to the tubes, crossed by air either due to natural or fan-forced ventilation.
The fans are typically installed on the outside near the coil, on the intake or outlet, or may be attached to the coil itself, as in the case of air-cooled evaporators.
Heat is usually exchanged by counterflow. Using the example of an evaporator, warm air first comes into contact with tubes carrying the “hotter” refrigerant leaving the coil, which is superheated and changes state to gas, and then the tubes carrying the colder refrigerant at the inlet, that is, in the liquid state before evaporating.
This optimises heat exchange, both whether the coil is used as an evaporator or a condenser.
Manufacturing technology adopts copper tubes and aluminium fins.
The tubes are bent to form a ‘U’ and then placed into the finned coil. Plastic deformation of the tube against the fins (expansion) creates a joint and thus thermal contact between the tubes and fins.
The tubes are free at one end and connected together using brazed-welded copper curved sections, so as to make a circuit in which the refrigerant flows, from inlet to outlet, configured on a case-by-case basis by the designer.
This in fact affects heat exchange capacity or the possible formation of frost on the coil.
As regards operation of the refrigerant circuit, finned coils are typically not very efficient heat exchangers and require high volumes to achieve high heat exchange capacities.
This means that the time the refrigerant remains inside the coil is quite long, with negative consequences on superheat control, for instance, which requires suitable management. The evaporator is in fact located between the expansion device and the sensors that control operation.
Another inherent problem with evaporator coils is the possible formation of condensate and consequently frost and ice on the surface, due to low temperatures of the tubes carrying the refrigerant in contact with moist air. This brings about the need to run defrost cycles (see this post for further details).
Tube bundle heat exchangers
These consist of a compact set of tubes secured at the ends (by expansion or welding) to two generally circular tube sheets; this bundle of tubes is housed inside a cylindrical body, called the casing or shell. Two special plenums distribute a fluid through the tubes (inside fluid), while another fluid flows over the tubes, inside the shell (outside fluid). Often the shell features baffles perpendicular to the tubes for the purpose of increasing turbulence in the outside fluid and thus achieving higher convective heat transfer coefficients. The baffles also help support the tubes.
Tube bundle exchangers are normally used as evaporators on water chillers: refrigerant flows through the bundle and water inside the shell. More rarely they can be used as condensers.
Flooded evaporators
To maximise the heat transfer efficiency of a tube bundle evaporator, flooded versions are often used, reversing the circuits so that water flows through the tubes and refrigerant enters the shell from below and then leaves from the top, possibly in the gaseous state.
Efficiency is high because the heat transfer surfaces are fully exploited, nonetheless refrigerant control is complicated by the fact that margins are very tight, and indeed a slightly excessive quantity will be returned as liquid into the compressor.
For this reason, expansion device control systems are adopted, involving expensive level sensors.
A tube bundle heat exchanger is preferred to plate exchangers where very high cooling capacities are required, and the cost and manufacturing complexities of the latter make them a poorer choice.
Plate heat exchangers
These are heat exchangers that are mostly suitable for use with liquids.
They consist of a series of rectangular metal plates pressed together and housed in a frame, or braze-welded in induction furnaces. The plates are corrugated in the centre and have four circular openings on the corners. The corrugations, as well as making the plates stiffer (the metal is 0.5-1.2 mm thick), allow the formation of a series of fluid flow channels between the plates when these are pressed together.
The circular openings on the corners of the plates, when these are pressed together, form the inlet and outlet ports for the two fluids. This means that the two fluids flow though the channels in very close contact, consequently giving a high heat exchange capacity in a very small volume, without ever mixing.
Like tube bundle heat exchangers, plate heat exchangers are normally used as evaporators on water chillers, or as condensers on water-source heat pumps, in particular home appliances.
Compared to tube bundles, plate heat exchangers are more compact and efficient, as well as more expensive and have a production process that does not allow very high cooling capacities to be reached.
Concentric tube heat exchangers
These are made up of an outer tube and one or more inner tubes. In co-current heat exchangers the two fluids enter from the same end, while in counter-current configurations the fluids enter from opposite ends. The latter configuration is generally more effective in terms of heat transfer efficiency and can ensure cold fluid outlet temperatures that exceed the outlet temperature of the hot fluid, something that’s not possible for co-current configurations.
In refrigerant circuits these are almost exclusively used as heat recovery heat exchangers, as currently they cannot ensure significant efficiencies.
Some examples include heat exchange between hot refrigerant gas leaving the compressor and water for central heating or domestic hot water use, or alternatively between liquid refrigerant leaving the condenser and superheated gas entering the compressor.
Climate Systems provides fresh commercial HVAC innovations to the Sioux Falls area. Call 605.334.2164, email info@climatesystemsinc.com or visit climatesystemsinc.com to learn more.
Source: “Heat Exchangers in the HVAC/R Sector,” carel.com
