If you have ever waited in anticipation for something, and it cannot happen soon enough, you know the meaning of the expression, “A watched pot never boils.” Unless you are a superhero that can create light amplification by the stimulated emission of radiation (laser) through your eyeballs, the pot of water would never boil by staring at it anyway. Boiling is defined as the “rapid vaporization of a liquid,” which can happen at various temperatures for various liquids. When it comes to Plumbing and HVAC design boiling fluid is what keeps our systems rolling, from heat transfer to refrigeration. Sometimes boiling fluids can cause unexpected problems as is the case with pump cavitation.
Most of us should know that water boils at 212° F. At least, we should understand that one way to get water to boil is to add heat. If you have gotten this far in life without boiling water for a hard-boiled egg or some top ramen noodles, you may want to check your privilege meter. When water boils at atmospheric pressure, otherwise known as the pressure humans live comfortably at, it turns to steam. Steam is basically water in gaseous form and acts as a reservoir for thermal energy. We can capture steam under pressure and move it around in pipes. You guessed it, when the steam moves to where we want it, so does the thermal energy associated with it.
The amount of energy carried around by the steam is generally around 1,150 Btu per pound of steam. Now that you know how much energy the steam is carrying, you can figure out how much steam you need depending on whether you are heating some air through a radiator or some water for domestic water heating. The fancy terms associated with this process are enthalpy of vaporization and enthalpy of condensation. Enthalpy of vaporization is the heat that goes into the fluid when it’s vaporized, and enthalpy of condensation is the heat that comes out of the fluid when it condenses.
As steam exists, typically a blend of hot liquid and gaseous water, it changes state as it does its work. As engineers, we can control how steam will behave by designing systems that affect steams temperature, pressure and state. If you take the time to study steam systems and look at steam tables, the other fascinating concept is how many cubic feet of space a pound of steam occupies. By conveying steam at higher pressures, we can compress it and deliver more of it to where it needs to go.
The expansion of liquid to gas and the compression of the gas back to liquid in a closed loop is how refrigeration is achieved. This is known as direct expansion, or DX. Examples of systems that boil water are your stovetop for making tea or a boiler to create steam in a heating system. What is clever about a DX refrigeration system is that it boils a “refrigerant” by utilizing the hot muggy summer air that nobody wanted around anyway.
As engineers, we can control how steam will behave by designing systems that affect steam temperature, pressure and state. By conveying steam at higher pressures, we can compress it and deliver more of it to where it needs to go.
In a closed loop boiler system, water is boiled to steam and then condensed back to water at atmospheric pressure once the steam has transferred its heat and done its work. As a natural occurring substance, water is comfortable changing state, similarly to the cycle that brings rain to the plains. Refrigerants used to control the air temperature in the spaces we occupy are manufactured and not intended to be released into the atmosphere. DX refrigeration systems control the temperature of the refrigerants by controlling the pressure and then blowing air over the network of pipes that convey the refrigerant. These networks of pipes are called evaporator coils (where the refrigerant is boiled) or condensing coils where the refrigerant is brought back to a liquid state.
One of the most fascinating applications where water boils is when it boils at room temperature. I would not have believed it myself if I did not see it with my own eyes. At a room temperature of 70°, water will remain in a liquid state as long as the absolute pressure in the room stays above about 0.4 PSIA. This is close to perfect vacuum, so there is not a lot to worry about. Now, say we try to pump that water with a suction lift through a dewatering hose. We may be familiar with the concept of Net Positive Suction Head Available (NPSHa). Atmospheric pressure provides about 33 feet of head to help get fluid into a pump. If you are attempting to “lift” that fluid from a pit, you would subtract that change in elevation. Then, you would also need to subtract the friction loss designed in the suction line. The last part of the equation is the Net Positive Suction Head Required (NPSHr) of the pump.
A poorly designed suction network on a centrifugal pump could cause the pump to cavitate. One can see how suction lift and friction loss through the network brings the system design to a point where the Net Positive Suction Head Required is not satisfied and fluid vaporizes. Once the fluid-vapor mixture enters the pump and gets pressurized, the vapor bubbles collapse and can cause damage to the pump. This sometimes becomes evident as a “pinging” sound. Cavitation is part of the reason that suction lift is not allowed for centrifugal fire pumps. It is also a reason why you might not want to put your backflow device on the suction side of the pump.
The above explanations are intended to be general in nature so you can learn about something you may not have known. In fact, if you have read this from beginning to end at a rate of about 200 words per minute, you would have been able to boil enough water for nice cup of coffee or tea and not had to watch it boil.