HYDRONICS for PLUMBING ENGINEERS
By roy c.e. ahlgren
What’s the Pressure?
“What’s the pressure?” sounds like an easy question. The pressure is the number you read on a pressure gauge, and pressure gauges are installed throughout a typical hydronic system to help monitor system performance and identify problems. They are installed at the boiler, on the suction and discharge of a pump, and across the coils in an air handler. While it’s easy to determine the pressure at any of these points, to determine if the system is operating properly, you also must know what a particular gauge is supposed to read. A pressure gauge in a closed hydronic system always reads the sum of as many as three “layers.”
Pressure is the force exerted over an area, and a column of water exerts a force proportional to its density and its height. FIRST LAYER: STATIC PRESSURE If the pump is turned off in a closed system, the water will not flow, and the pressure gauges read the static pressure. (Note: Gravity-induced flow could occur when the pump is off if a significant temperature difference exists from the top to the bottom of the system.) Pressure is the force exerted over an area, and a column of water exerts a force proportional to its density and its height. Water density is determined by the concentration and type of additives such as treatment chemicals and antifreeze. It also is affected by temperature. Like most things, water expands and becomes less dense as it is heated. For example, if you consider water with no additives at a temperature of 60°F, a column 2.3 feet high will exert a pressure of 1 pound per square inch (psi). Using this fact, you can derive the following rule of thumb: With the pump off, the expected gauge reading should be at least the height of the system above that gauge in feet divided by 2.3, plus the gauge reading at the top of that column.
SECOND LAYER: OPERATING PRESSURE Of course, 60°F water isn’t very useful in heating a building, so you must raise its temperature. Thus, the water expands, adding another static layer to the pressure gauge reading. As the water heats and expands, so does the system piping. The net expansion could be defined as the expansion of the water minus the expansion of the piping as both heat up to the operating temperature. In a typical heating system, the net expansion might be 3.5 to 4 percent, which means that a system that had only 100 gallons of cold water now has 104 gallons of hot water. 4 Plumbing Systems & Design
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The air cushion trapped in the compression tank absorbs the net expansion, and as the volume of air decreases, its pressure increases according to Boyle’s law, which states that at a fixed temperature, pressure and volume are inversely proportional (while one doubles, the other halves). Every gauge in the system will register this increase in static pressure because the water isn’t flowing. If you want to achieve the required flow rate, the pump must create and maintain a differential pressure.
THIRD LAYER: PUMP HEAD The pump applies work to a pound of liquid at the suction nozzle, discharging it at a higher pressure. This differential pressure acting on the water, which is essentially incompressible, overcomes friction in all of the system’s components, providing the flow required to move enough heat to keep the people inside the building comfortable. The pressure effect of the pump is reduced by friction, so the pressure layer caused by the pump will be smaller as it overcomes friction in pipes, fittings, coils, and so on. At the point where the compression tank connects to the system (the point of no pressure change), the pump effect is zero. That’s why it’s a good idea to install the compression tank near the pump suction. By the time the liquid returns to the pump suction nozzle, all of the pump head is used up— that is, it has been converted to thermal energy by the system friction.
SO WHAT’S THE PRESSURE? The answer depends on several things: • Where the gauge is located: Is it at the top or the bottom of the system? • When the reading is taken: Is the system hot or cold? • Is the pump on or off? • How far is the gauge from the pump and the point of no pressure change?
Roy Ahlgren is a consultant to the hydronics industry. He
served as chair of the ASHRAE Technical Committee on Hydronic and Steam Systems and was the director of the Bell & Gossett Little Red Schoolhouse. For more information or to comment on this article, e-mail
[email protected].
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