Wort Boiling Systems
Boiling is the most energy intensive operation in the brewery. Although steam is by far the most applied method of imparting heat to brewing operations, other heat transfer methods used include direct-fire gas, electric immersion elements, and even pressurized hot water. Electricity is mostly used by breweries with limited production, i.e., home brewers. A British thermal unit (Btu) is a measure of the heat content of fuels or energy sources. It is the quantity of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit).
The length of time taken to heat wort to its boiling point can be reduced if the wort is preheated during transfer from the separation stage to the kettle using a plate and frame heat exchanger (Figure 10.3).
Traditionally, wort was boiled in direct-fire kettles, often made of copper. These copper kettles tend to have a concave bottom or shielding that can trap the heat and keep it in contact with the bottom of the vessel to improve heat absorption by the kettle. The burners are generally housed in a cast iron combustion chamber underneath the kettle distributing heat across the kettle bottom. It can be one main flame or more commonly several flames allowing for better heat distribution across the kettle. A newer design “pushes heat through a coil” inside the kettle for more efficient heat transfer, as the coil is immersed in the liquid.
Steam provides process heating, pressure control, mechanical drive, and component separation, and is a source of water for many process reactions. Steam has many performance advantages that make it an indispensable means of delivering energy. These advantages include low toxicity, ease of transportability, high efficiency, high heat capacity, and low cost with respect to the other alternatives. Steam has the capacity to deliver a large amount of heat quickly to a point of use without the need for additional pumping.
Kettles with External Heating Jackets
Kettles with heat jackets are very common and are usually the next step after direct fired kettles. In a jacketed kettle, a hot fluid—usually steam—is conducted through an enclosed space on the outside of the kettle and brought into contact with the bottom and sides of the kettle. Steam operates at pressures of 2- to 4 bars absolute (15–45psig). The heating jackets are symmetrically arranged around the vessel, or the jackets may be placed asymmetrically in order to encourage a rolling boil.
Kettles with Internal Calandria
An internal calandria is a shell and tube heat exchanger mounted inside the kettle allowing for more efficient heat transfer. The unique feature is that the tubes are open at the upper end or both ends to allow wort to flow in and out. Steam is provided to the shell to boil the wort. The wort is propelled upward by the convection current created by steam flow, in which case both ends of the tubes are open, or it can be pumped. Intermediate and combined systems using mechanical pumping as well as steam-driven pumping are in use. Usually, the output of the calandria shoots up through a constricting tube (sometimes called a venturi tube) and emerges above the level of the wort striking a deflector plate that breaks up the liquid into droplets (Figure 10.4).
Kettles with External Calandria
Many breweries have an external boiler outside of the kettle, i.e., calandria, through which wort is pumped. The external calandria normally is made up of tube type or plate type calandria. The most common type of external wort boiling system is the tube heat exchanger system (Figure 10.5). These tubes provide an enormous surface area on which vapor bubbles can nucleate, and thus provides for excellent volatilization. Wort is heated by pumping it from the bottom of the kettle through the external wort boiler and back into the kettle through a vertical tube called a “fountain”.
Merlin is a new type of boiling system designed as a boiling and stripping system. The wort is pumped from the lauter tun into the whirlpool over the conical heating surface of the Merlin as a thin film. It comprises a vessel containing a conical heating bottom for boiling and evaporating wort. The whirlpool, below the Merlin device, serves as a collection vessel for wort. A pump circulates wort between the vessels. Wort is lautered into the whirlpool, at the end of which the wort is pumped as a thin film over the conical heating surface of the Merlin.
Click on the following topics for more information on wort boiling.