Direct Contact Heat Exchangers

Direct contact heat exchangers are those which use the liquid-vapor phase change of working fluid inside the heat exchanger and energy generation systems including the heat exchanger. The heat exchanger is specifically helpful in blend with solar energy receiver. The heat exchanger comprises an insulated pressure vessel packed with pebbles or spheres. Working fluid in the vapor phase from a solar heat source comes into the unit and condensers transferring thermal energy to the pebbles.

The mode of operation

The liquid level of working fluid about the pebbles is reduced during this mode of operation. When it is desirable to discharge this stored energy the liquid level is raised and the energy is transferred to the working fluid which forms a vapor and can be used to drive a turbine or other prime movers.

In other words, direct contact heat exchangers include heat transfer between hot and cold streams of two phases in the absence of a separating wall. Hence such exchangers can be categorized as gas liquid, immiscible liquid and solid liquid or solid gas.

The application

Many of the direct contact heat exchangers come under the gas-liquid category where heat is transferred between a gas and liquid in the form of drops, films or sprays. Those types of heat exchanger are used predominantly in air conditioning, humidification, water cooling and condensing plants.

And a direct contact heat exchanger storage system involves certain factors. For instance, one of the widest uses of heat exchangers is that of air conditioning of buildings and vehicles. This class of heat exchangers is usually known as air coils o just coils since their serpentine internal tubing. Liquid to air, air to liquid HVAC coils are particularly altered cross flow arrangement. In vehicles, heat coils are frequently known as heater coils.

Some of the examples, where it is used

On the liquid side of these heat exchangers, the general fluid is water, a water-glycol solution, steam, or a refrigerant. For heating coils, hot water and steam are the most common and this heated fluid supplied by boilers. For cooling coils, chilled water and refrigerant are most usual. Chilled water is supplied form a chiller that is potentially located very far away, but refrigerant must come from a nearby condensing unit. When a refrigerant is employed, the cooling coil is the evaporator in the vapor solidity refrigeration cycle. HVAC coils which use the direct expansion of refrigerants are usually known as DX coils.

Questions:

• What is direct contact heat exchanger?
• State the significance of direct contact heat exchanger.