What is a plate heat exchanger?
A plate heat exchanger consists of a package of thin corrugated metal plates with openings to allow the passage of fluids between them.
The plates are arranged in such a way that between neighbouring plates a channel is formed. Every second channel is open to the same fluid. The actual heat transfer takes place between two adjacent channels, from one fluid to another; for example, hot water to milk or stem to hot water.
The corrugation of the plates supports them against differential pressure and creates turbulent flow in the channels. In turn, the turbulent flow provides a high heat transfer efficiency, making the plate heat exchanger very compact when compared with a traditional shelland-tube heat exchanger.
Depending on the method used to seal the plates, plate heat exchanger types are divided into three groups:
- Gasketed plate heat exchangers (or plate & frame heat exchangers)
- Brazed plate heat exchangers
- All-welded plate heat exchangers
History of Gasketed Plate heat exchanger
The gasketed plate heat exchanger was invented in the early 1930’s. In those days it was primarily used in the food industry, where a heat exchanger that was easy to clean was demanded to meet hygiene requirements. The first steam heated plate heat exchangers were launched during the 1940’s, serving as milk pasteurisers. Over the years, the plate heat exchanger concept has proven successful and it has evolved into a new range of product innovations. In the late 1970’s, the brazed plate heat exchanger was launched, which uses vacuum brazing techniques to seal the plates.
The next step in the development process was taken in 1994, when the all welded plate heat exchanger was introduced. Here, the plates are laser welded, forming a very strong construction.
Today, the plate heat exchanger is used in a multitude of duties in chemical processing, HVAC, food processing, marine / off shore applications and refrigeration.
The gasketed plate heat exchanger
In a gasketed plate heat exchanger (frame & plate heat exchanger), the plates are fitted with elastomeric gaskets which seal the channels and direct the fluids into alternate channels. The plate pack is assembled between a frame plate and a pressure plate, and compressed by tightening bolts fitted between these plates. The channel plates and the pressure plate are suspended from an upper carrying bar and located by a lower guiding bar, both of which are fixed to the
support column. The physical design of the gasketed plate heat exchanger allows easy cleaning, and modification of capacity by the addition or removal of plates.
The gaskets
The most common gasket type in steam heaters is the glue-free Clip-on gasket.
Glue-free gaskets are easy to change and have a longer lifetime than glued gaskets.
Glued gaskets are preferred when the duty requires frequent mechanical cleaning of the plates. The glue will keep the gasket in the same position regardless of how many times the unit is opened, which reduces the risk of leakage. Furthermore, the gasket will not fall off if the plates are cleaned by means of a high pressure nozzle or manual brushing.
The most common gasket material used in steam heaters is EPDM, and is normally preferred when the heated medium is water or any aqueous fluid. EPDM can handle a steam temperature up to 160°C.
Nitrile rubber is used when heating mineral oils and other petroleum products. It is suitable for temperatures up to 140°C.
Fluor-elastomers are preferred for duties where acids or other aggressive fluids are to be heated, or when the steam temperature is up to 180°C.
The channel plates
Various channel plate styles are available, corrugated in different patterns, suitable for a large number of fluids and a wide range of thermal duties.
The most commonly used plate material is stainless steel, AISI 316. Titanium plates are used for media containing chlorides e.g. sea water. Exotic materials such as Hastalloy, Incoloy and graphite are available for chemical processing duties.
The thickness of the plates is normally in the range 0.4 – 0.7 mm.
The gasketed plate heat exchanger as a steam heater
Because of its flexibility, the gasketed plate heat exchanger is used in a great number of applications in the HVAC and other industries. It is usually the temperature performance of the gaskets that sets the limit of its use. Their elastic mechanical design makes them resistant to pressure pulsation and thermal fatigue.
This unique feature makes the gasketed plate heat exchanger the ideal solution for batch operations and instantaneous water heating, or other duties where the heat exchanger will be exposed to thermal cycling.
The brazed plate heat exchanger
The brazed heat exchanger is a variation of the traditional gasketed plate heat exchanger.
It evolved from the conventional gasketed plate to answer the need for a compact plate heat exchanger, suitable for high pressure and high temperature duties.
The brazed plate heat exchanger is, like the gasketed PHE, constructed of a series of corrugated metal plates, but without gaskets, tightening bolts, frame etc.
The brazed plate heat exchanger simply consists of stainless steel plates and two end plates. All the plates are brazed together in a vacuum furnace to form an extremely compact and pressure/temperature resistant heat exchanger with superior heat transfer characteristics. Copper is the usual brazing material.
Nickel is used in its place where the heated media contains ammonia, which would dissolve copper, or, in applications where the presence of copper ions is undesirable.
Unlike the gasketed plate heat exchanger, the brazed plate heat exchanger cannot be disassembled. The scrubbing action of the highly turbulent flow retards fouling. If cleaning is required, the unit can be back-flushed to prevent clogging, or chemically cleaned to eliminate the build up of scale.
The brazed plate heat exchanger as steam heater
The robust design of the brazed PHE makes it suitable for higher pressure and temperature applications, but is also a disadvantage, as the many contact points between the plates make the heat exchanger prone to thermal fatigue.
This means that special attention must be paid to the application it is used for, and the control method which is applied.
Generally, applications where the brazed plate heat exchanger may be exposed to large, sudden, or frequent changes in temperature and load must be avoided. An example of such a duty is instantaneous tap water heating with steam. Space heating is a suitable application for the brazed plate heat exchanger, provided that the proper control method is applied and the components in the control system are correctly sized.
Primary steam control with active condensate drainage is the preferred method, as this system will minimise the risk of thermal cycling, as long as the heat exchanger and the control components are correctly matched. Any type of on/off control would almost inevitably lead to thermal stress and must not be applied.
The brazed plate heat exchanger is a very price competitive alternative to other heat exchanger types. Its primary uses are for space heating and applications where it is not exposed to sudden temperature changes.