Exponentialized

Yeah, you are not going to find the word exponentialized on a Google search (the new way to verify a word’s legitimacy). Back in the day before computers, we looked in a dictionary to see if a word was real or settle an argument during a Scrabble game. The suggested word when I checked was exponentialize, which according to Wikipedia means to increase or magnify considerably.

I have always liked the line in the late 1960s song Time Has Come Today, “And my soul has been psychedelicized.” It was the Chambers Brothers’ biggest hit single in 1968, a call-to-action protest song about civil rights and the end of the Vietnam War. Turns out the word “pschedelicized” was not a real word either, but popular at the time. Loosely translated from the Urban Dictionary, it means to alter your perception, usually under the influence of a drug.

Late into one of my recent Nordic Track workouts, when the endorphins were kicking in and I was thinking about the subject of my next article, I rhymed the word psychedelicized with exponentialized. Willie Chambers rhymed it with tumbling tide. My rhyme won’t make it into any poem or country song, but it has stuck in my brain ever since and I want it to stick in yours.

Next time you are thinking about water flow in a closed loop hot water or chilled water piping system, think of exponentialized, because that is what happens to the resistance to flow in a pipe as you increase the flow rate. The resistance doesn’t just go up, it increases considerably.

This resistance to flow is very predictable and quantified by the measurement called feet of head here in the US and meters of head in the rest of the world. As water moves through the pipe, it has to overcome the friction caused by the flow. Our logic predicts in our brain that if we double the flow, the head would double. However, that’s not what happens in the real world.

If we have 5 gallons per minute flowing in a 1” pipe, it creates about 2 feet of head per 100 feet of pipe. When we double that flow to 10 gallons per minute in that same 1” pipe, it creates about 7 feet of head per 100 feet of pipe, not our predicted 4 feet. If we triple the flow to 15 GPM, the head jumps to 15 feet, not 6 feet. The head created has been exponentialized.

Of course, water doesn’t always flow through pipes. It encounters lots of other things, like ball valves, boiler heat exchangers or chillers, air separators, pumps, control valves, radiation and sometimes who knows what. Some things create more pressure drop than others, while the pump creates the pressure differential to make the water move through the whole mess.

Ball valves can be full flow, which means their resistance is about the same as a piece of pipe of the same size. Ball valves can also be reduced flow, which means the water passes through a port that is smaller than the pipe size, which means it has more resistance, which increases exponentially as the flow increases.

Control valves will have a published rating that is used to predict pressure drop at different flow rates. That rating is called the Cv relationship or flow valve coefficient. Pretty technical terms for us non-engineer types. Basically, the higher the number is, the lower the resistance is. To continue with our example, let’s look at a 1” control valve.

The typical 1”, 24-volt zone valve we stock has a Cv rating of 5.0. At 5 GPM, it creates a resistance to flow of about 2 feet. That’s not much and most pumps won’t have a problem moving water through it. Let’s double that flow to 10 GPM through the 1” valve and now the head moves up to about 9 feet. That 9 feet then gets added to the rest of the resistance in the system and you need a high head pump to get the water flowing correctly. Triple the flow and the head gets exponentialized to about 20 feet.

Be real careful now with your pump selection. It’s likely that you will be spending more money on a larger pump than if you selected a valve with a higher Cv rating. If you need that 15 gallons per minute of flow, select a control valve with a Cv rating closer to 15 and you’ll get a resistance to flow closer to 2 feet.

Back to the real world and how it may affect you. A scenario I can see is a multiple-zone valve system where the installer wants to make the installation look great. All the pipes are flush and plumb and all the zone valves are the same size. However, all the zones have different heating requirements. Let’s say this is an older house remodel. Most of the zones are smaller areas like baths or bedrooms, but the original house zone is still quite large.

The installer picked ¾” as the zone valve size, since that’s the size he sees all the time, but it has a Cv rating of only 3. If the large zone is 120,000 btus, that means the flow rate for that zone should be 12 gallons per minute at the typical 20 degree temperature drop used by the industry for decades. Trying to flow 12 GPM through that zone valve with the Cv rating of 3 would result in a head loss of 30 feet, just for the valve. That’s a lot of head.

The pump can’t develop that much head, so the flow rate decreases and the temperature drop increases. If it is piped as primary-secondary, the hot water from the boiler that is supposed to go to the zone goes back to the boiler, which will quickly short cycle off on high temperature without satisfying the thermostat for the main house zone. The homeowner is thrilled.

When you have the ability, size control valves for low head. If you have a situation with low flow, look around for an obstacle in the way that could create high head, like an under-sized control valve.

Patrick Linhardt is a thirty-seven-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.