Graphite is a corrosion resistant material of construction which gives no ion pick up, hence it is suited to applications mainly within the fine chemical pharmaceutical and fine chemical industries.
Although cylindrical (modular) and shell and tube graphite heat exchangers are still manufactured, the cubic block is much more compact, less wasteful of materials and more easily maintained. However, each type still has its particular niche, the cubic form being mainly used in conjunction with reactor vessels as condensers.
For condensing applications, the sub assembly (see Figure 1 on page 533) is usually made of a laminated construction so that slots can be made on the process side. A slot increases the available area for condensation by a factor of two for the same unit volume, which is particularly advantageous.
The header on either the process or the service side can be manufactured to contain a large number of baffles (e.g., 80 passes can be achieved in a unit having just 13.94 m of heat transfer area). This is at the detriment of pressure drop and it is advisable not to have a liquid flow-rate of much more than 1 m/s through the exchanger.
Graphite itself is a porous material, particularly when being used in the construction of heat exchangers and bursting discs, needs to be impregnated with a resin. There are two basic resins, furanic and phenolic based. These resins help form an impermeable barrier between process and service sides. Sometimes they are not compatible with chemicals, particularly highly oxidizing agents, caustic above 15% concentration and wet halogen gases; it is advisable to contact the manufacturer (for advice) on compatibility.
Graphite cubic blocks are made for a wide range of applications, for example:
To replace glass. Graphite is a more thermally efficient material of construction, typically using 1/2 to 1/3 the heat transfer area required by a glass unit. Graphite is more robust and can be subjected to greater pressure.
In sulfuric acid dilution. A small mixing chamber can be attached to the process inlet connection where sulfuric acid and dilution, water are mixed and then instantly passed through the exchanger.
In vacuum pump exhausts. To condense vapors before and after vacuum pumps (Busch and Rietschle particularly).
The major advantage of a cubic unit is its compactness and ability to cope with a very large number of chemicals and duties up to 20 bar pressure and condensing duties at temperatures above 165°C. In addition, its form of construction makes it easily accessible for maintenance.