The Difference a Pitot Tube Can Make — Part 1
Robust design results in near-zero maintenance
Everyone involved with IAQ knows about the 1970s, when excess energy use became Public Enemy No. 1 for commercial building designers. Saving energy was the critical goal: Air leaks were being hunted like wild animals, and windows and doors were sealed so tightly that not a breath of conditioned air could seep out.
And the fuel economizer, the behemoth that recirculated conditioned air and milked every possible Btu from it, became the hero of the day.
The only problem was that the recycled air made the building occupants sick because the pollutants in the air were getting trapped in the building, increasing over time.
So — back to the drawing board.
Engineers soon had the problem in hand. The new solution was to introduce and maintain the right amount of outside air to keep pollutants at a safe minimum. The theme was, “the solution to pollution is dilution.”
They quickly developed formulas based on all of the building variables they could identify. Ideal amounts were determined for individual buildings. All that they had to do was install flow meters for the supply and return air, and program the BMS to monitor and control the amounts of outdoor air.
But, there was one more unforeseen problem: airborne particulates.
“Airborne particulates have become the bane of today’s facility managers in commercial buildings,” said Mark Calkins, regional sales manager of Paragon Controls, a leader in Pitot tube air flow meters. “And that’s not because the particulates aren’t manageable; they are. The problem is that managing them comes at a cost — often a high one. The most crippling cost is the maintenance challenges involved in dealing with particulates once they start interfering with an outside air system. Most airflow meters installed with outside air systems rely on thermal dispersion technology and involve heated sensors that attract and accumulate airborne particulates. That accumulation degrades meter performance and requires frequent manual cleaning, often resulting in damage to the meters.”
The Pitot Tube Solution
Fortunately, there is an alternative flow meter technology that provides accurate airflow measurement and resists particulate contamination: the Pitot tube.
Originally invented by French engineer Henri Pitot in the 18th century, it has evolved to become the standard for measuring airspeed in planes and as a speedometer for boats, among many other uses. Since the Pitot tube operates effectively in these particulate-rich environments, it has quickly become a primary choice in airflow measurement.
“A Pitot tube is used to measure fluid flow velocity,” said Calkins. “The tube is pointed into the flow, and the difference between the stagnation pressure at the tip of the probe and the static pressure at its side is measured, yielding the dynamic pressure from which the fluid velocity is calculated using Bernoulli's equation. A volumetric rate of flow may be determined by measuring the velocity at different points in the flow and generating the velocity profile. Typical accuracy is 2 percent or less.”
Since the sources for airborne particulates are far more numerous and varied than most facility managers are aware of, it will be helpful to examine how the meters perform in some challenging environments.
Al Fenderson is the professional facility manager responsible for maintaining air quality at a modern Midwest hospital complex.
“When I came here, the hospital was relatively new, as were the metering capabilities and the building management system,” said Fenderson. “So I didn’t anticipate any serious airflow problems.”
In hospitals, airflow management is difficult enough to manage without other problems being introduced. That’s because it requires a careful mix of positive and negative airflow, depending on the functions of each department. For instance, areas involving patients with infectious diseases require negative flow, so the diseases don’t spread to the rest of the facility, while operating rooms need positive flow to ensure that fresh, clean air is always present.
Farm Dust Particulates
“My problems started soon after I arrived,” said Fenderson. “It became clear that the BMS wasn’t managing the outside air supply to the hospital as efficiently as it was supposed to. My team and I immediately began looking for the cause. The first place we examined was the flow meters used to monitor the volume of outdoor supply air and return air from the facility. They were thermal dispersion meters, the most common type in use at the time, and were mounted strategically on fan inlets and outside air intakes.
The problem turned out to be that soil particulates were attracted to the heat the sensors produced and coated them, gradually degrading the meters’ efficiency. =
“We have farms all around us,” said Fenderson. “Whenever they do farm work, they kick up dust. It’s something we’re very sensitive to because dust from the Earth carries a variety of harmful bacteria and other pollutants that we cannot expose our patients to. That’s why the building has a comprehensive filtering system to assure the quality of our air.”
This was the first time Fenderson’s team realized the particulates and the pollutants they carried were a problem for outside air management. And if they couldn’t accurately control the outside airflow, they became vulnerable to introducing dangerous outside particulates into the hospital environment.
The only quick solution for the particulate problem was an accelerated maintenance program that involved manually cleaning the sensors.
“With all of the flow meters we had, I knew the maintenance would be costly in man hours,” said Fenderson. “What I didn’t know at the time was the expense we would face in damage to the thermal meters themselves. It turns out that the glass that covered sensors broke easily, and, since the meters were often in difficult-to-reach locations, many would often crack during cleaning and need to be replaced. That’s when we started aggressively looking for a new airflow measurement solution.”
The search led to a Pitot technology company that specialized in airflow measurement.
“What they showed us made sense regarding particulates, so we decided to do a comparison study,” Fenderson said.
“The hospital had a few thermal dispersion meters that had been damaged, so we replaced them with Pitot airflow meters and stepped back to let Al and his team do their own comparisons,” said Calkins.
“There was no degradation of signal due to particulates or anything else, and the accuracy was as promised,” Fenderson said. “We were so pleased with the results that we are having Paragon replace all of our thermal dispersion meters, and we have already committed to them for our planned new addition.”
Publication date: 3/18/2019