Dishing About Plate Heat Exchangers
Although several types of plate heat exchangers are offered, this article focuses on standard gasketed plate-and-frame heat exchangers. The main components of the plate heat exchanger include:
- Plate Pack. The plate pack is composed of a number of corrugated heat transfer plates according to the heat transfer surface required. The plate corrugations provide support between plates and encourage turbulence, which gives the plate heat exchanger its high heat transfer.
- Gaskets. The molded elastomeric gaskets on the plates ensure that the flow channels are sealed securely from each other, preventing cross-contamination. The gaskets also determine the direction of flow within the exchanger.
- Frame. The frame provides structural support and pressure containment by enclosing the plate pack. The main frame components consist of a fixed end, a movable end, upper and lower carrying bars, and tightening bolts.
- Connections. The connections for incoming and outgoing media usually are found in the fixed end plate. In the case of a multiple-pass design, connections also are found on the movable end.
Knowing these components will help you understand why a plate heat exchanger might or might not be suitable for your process application.
How to SpecifyThere are seven main variables that can be used to specify a plate heat exchanger. In general, you must know the values for at least five variables to specify a heat exchanger. On the primary side, the variables are temperature in, temperature out and flow rate. On the secondary side, the variables include temperature in, temperature out and flow rate. The seventh variable is the overall heat transfer. By providing five of these variables, the other two can be determined.
Physical Properties. There are five physical properties that are required to be known for each fluid: viscosities at the inlet and outlet, thermal conductivity, specific gravity and specific weight. For standard fluids with known properties, these properties do not need to be specified.
Particulates. Standard plate heat exchangers generally do not handle particulates in excess of 0.5 mm. Free-flowing-style plates are available that can handle larger particulate sizes. For applications with larger particulate sizes, consult with the plate heat exchanger manufacturer for specific recommendations.
Most, if not all, American plate heat exchanger manufacturers can provide their units with an ASME U stamp.
A Plateful of AdvantagesThere are several advantages to using plate heat exchangers, including:
Cost Savings. Plate heat exchangers save costs with their high efficiency, low first cost, compact installation and ease of maintenance.
High Heat Transfer Coefficients. Plate heat exchangers provide high rates of heat transfer due to the turbulence-inducing corrugations of the plates. Different corrugations allow the plate heat exchanger to be tailored to specific heat transfer and pressure drop requirements. With water vs. water type applications, it is not unusual to achieve 4oF approach temperatures.
Safety. The plates' special gasket configuration prevents any mixing of the media being processed. In the port area, the fluids are separated by a double-gasket design with a vent to atmosphere, where the fluid will be released if one of the gaskets fail. In more extreme cases, a safety plate design can be provided. By welding two plates together, a safety barrier is provided so that one fluid drains to atmosphere instead of contaminating the adjacent fluid if a plate fails.
Flexibility. Plate heat exchangers can be adapted to changing process conditions by simply adding or removing plates. This eliminates the need for reinvestment.
Low Product Hold Volume. The small volume of liquid in the plate heat exchanger is the reason for the relative low weight of the equipment. It enables a faster startup and shutdown of the complete plant compared to conventional heat exchangers. Lower hold volumes also mean higher recovery rates for high value products.
Maintenance. Any maintenance of the plate heat exchanger can be carried out easily. Where a shell-and-tube heat exchanger requires a clear pull space equal to the length the tube bundle, a plate heat exchanger only needs a few feet clear on each side of the unit for all maintenance.
Materials. Plates are available in several grades of stainless steel, including 304, 316, 317, and in specialty materials such as Hastelloy, Incoloy, titanium, nickel and tantalum. The gaskets are available in nitrile rubber (NBR), butyl rubber (Butyl), ethylene propylene rubber (EPDM), silicon rubber and Viton. Standard frames are manufactured from carbon steel. For the food industry, the frames are available in solid stainless steel as well as carbon steel cladded with stainless steel. Some plate heat exchanger manufacturers will provide special unlisted materials of construction for just one particular customer.
Not for EveryonePlate heat exchangers are not the solution for every heat transfer need. Applications with aggressive products that cannot have any contact with elastomers are not appropriate. In those cases, another type of heat exchanger needs to be investigated. Particulates that range in sizes in excess of 12 mm generally cannot be handled by the current state-of-the-art in plate heat exchangers. Pressures in excess of 350 psig start to limit the available plates that manufacturers can provide.
The North American market has yet to fully mature with respect to plate heat exchangers. When compared with Europe, North America still uses the more conservative option of a shell-and-tube heat exchanger for applications that readily can be accomplished with the plate-and-frame heat exchanger. Any plate heat exchanger manufacturer would be willing to discuss your application to see if a plate heat exchanger can meet your needs and budget.