Building a steam boiler (2)

Your BOILER is a device having the simplest concept yet is often the most troublesome in your plant. In addition, it is also the most potentially dangerous. For this reason, NEVER DO ANY MAINTENANCE OR REPAIRS OF ANY KIND ON A BOILER THAT IS HOT OR UNDER PRESSURE.

There are many different types of boilers, usually differentiated by the method of transferring heat from the burning fuel to the water. Examples of various types are water tube, fire tube, and tubeless, with many configurations within each type. We will not get into the FABS and FAQS of each type since this issue aims to understand what a BOILER is and how a BOILER functions in generating steam.

Take a pot of water, put it on a stove, and heat it on a low flame. Put a thermometer in this pot of boiling water; if you are at sea level, the thermometer will read 212 degrees Fahrenheit. Turn the flame up all the way, and the water boils more violently and changes to steam at an accelerated rate, but the thermometer still reads 212 F. As long as there is water in the pot, that thermometer will stay at 212 F. Steam rises from the surface of the water and disperses into the air without being able to do much in the line of useful work except if you have some vegetables in a steamer above the water in the pot.

If you put a lid on that pot that was tight enough to contain the generated steam, the pressure in the pot would begin to rise. Since this increase in pressure is also pressing down on the boiling water, it takes more heat energy (see definition of BTUs from last month’s article) to force the water to convert to steam. This increase in heat energy is reflected in an increase in the temperature of the water steam mixture. The higher the pressure inside the pot, the higher the temperature of the steam and water mixture’s boiling point.

There is a direct and unchanging relationship between steam pressure and temperature; forgive the expression; it’s baked into the nature of water. That is why you heat your dry cleaning presses with 80 lb. steam (324 F.) and your laundry machinery with 100 lb. steam (337 F.). If you had some veggies in the pot, they would cook to mush in no time. What you now have is the kitchen appliance called a PRESSURE COOKER, capable of producing internal temperatures much greater than 212 F. In addition, the space above the water line and the lid is a built-in storage area for the contained steam. At some point, unless you somehow release the steam or turn off the heat source, the pressure will increase until the pot ruptures (explodes). Therefore, on all pressure cookers, there is a weighted or spring-loaded relief valve that controls the internal pressure and a safety or blow-out fuse or vent that will rupture if the relief valve fails. The following paragraphs will incorporate the concept described in the PRESSURE COOKER analogy to constructing a steam boiler.

Now let’s start adding components to a closed pressure vessel to create a gas-fired BOILER. Bold letters at the beginning of each of the following paragraphs refer to the component location on the accompanying illustration.

A. Let’s install an electric SOLENOID valve in the gas supply line so that we can turn the gas on and off; this would be your gas supply system, at the end of which is a burner that distributes the gas flame evenly around the pressure vessel.

B. Since we are dealing with gas and its explosive nature, we need to know that when the SOLENOID valve opens, the gas ignites almost immediately; if not, we shut the SOLENOID cutting off the gas flow to avoid a possible explosive condition. This control system is the ignition monitoring system.

C. As I previously mentioned, as long as there was water in the pot, the temperature would not rise above the boiling point of the water. When all the water boils away and there’s nothing to adsorb heat energy generated by the burning gas, the temperature of the metal of the pot will rise until it glows red. In a BOILER, this would destroy the BOILER. So, let’s place a sensor in the BOILER that monitors the water level. If the water level drops too low, we’ll shut off the gas supply to the burner. This sensor is your boiler’s LOW WATER LEVEL CONTROL. Assuring the proper water level is so critical there is usually a requirement for a SECOND LOW WATER LEVEL CONTROL control for safety.

D. Another parameter that needs to be monitored and controlled is steam pressure. Steam pressure regulation is accomplished by an OPERATING PRESSURE CONTROL that turns the gas SOLENOIDS on and off at a preset pressure when the desired pressure is reached. Usually, an additional SAFETY PRESSURE CONTROL is added as a backup measure. The SAFETY PRESSURE CONTROL activates when the maximum BOILER operating pressure is exceeded, shutting off the gas SOLENOID. The SAFETY PRESSURE CONTROL requires a manual reset, warning the operator of a problem in the PRESSURE CONTROL system. Steam explosions are devastatingly destructive. Therefore, an additional device is required to prevent an overpressure explosion. This device is the PRESSURE RELIEF VALVE. Should all the other PRESSURE CONTROL devices fail, this valve will open before the bursting pressure of the BOILER is reached, allowing steam to vent to the atmosphere, thereby lowering the steam pressure.

In the following several issues, I’ll explain other systems and components which when combined form the steam and return systems used in typical dry cleaning and laundry operations.