Heat exchangers are the devices used for heating or cooling solutions in the applications. The mechanism of transferring heat is done using material that is a good conductor of the heat. Metals like copper, nickel, steel, and aluminum are used widely for manufacturing these devices. Materials like ceramic and plastic are also used for this purpose.
Types of heat exchangers
These devices are available in different designs that include plate, plate fin, tube, tube fin, shell and tube, and double pipe types. Heat transfer takes place between two different mediums that can be liquid to liquid, liquid to gas or gas to liquid. Water and synthetic oil are good conductors of heat and are used widely as a medium for heat transfer.
In the liquid to liquid type of device, two liquids of different temperatures flow into different pipes or plates without coming into direct contact with each other. The two liquids are separated using the wall of the pipes or the plates, which is a good conductor of the heat. The heat transfer takes place between the hot liquid and the cold liquid and as a result, the hot liquid loses the temperature and the cold liquid gains the temperature. The directions of the flow of the hot and cold liquids can be parallel, opposite, or perpendicular to each other.
Depending on the directions of the flow of liquids, these devices can be classified into parallel flow, counter flow, and cross flow types. In the parallel flow type of device – cold and hot liquids are flowing in the same directions, in the counter flow type of device – hot and cold liquids are flowing in opposite directions and in the cross flow type of device – hot and cold liquids are flowing in perpendicular directions to each other. Each type of device has its advantages and disadvantages.
Cross Flow heat exchangers
In the cross flow heat exchangers hot and cold liquids move in perpendicular directions to each other, so when the device size is small it becomes easy to locate inlets and outlets of the hot and cold liquid. Cross flow heat exchangers are considered more efficient than the parallel flow heat exchangers and LMTD (Logarithmic Mean Temperature Difference) of the cross flow heat exchanger is higher than the parallel flow heat exchanger.
For the specified temperature of the inlets and outlets, at a specified flow rate, required flow area for the parallel heat exchanger is maximum but the required flow area for the cross flow heat exchanger is lesser. This is the reason why cross flow heat exchangers are preferred over parallel flow heat exchangers. Compared to the counter flow type of device, for the cross flow type of device, it is easier to provide inlet and outlet connections and it is possible to keep the design compact.
Mixed cross flow type
In the shell and tube type of heat exchanger, if the flow of the liquid in the shell is in the perpendicular direction of the tubes, then it is called mixed cross flow heat exchanger. Heat transfer rate increases for this type of mechanism because of the turbulent flow.
Unmixed cross flow type
In this type of device, there are fins attached to the pipes and liquid flows over these pipes. Heat transfer rate increases because of the finds but attaching fins increases the cost of the device.