Heatin’ Up! (The Lafayette College Pool) by George, Brian, and Erich
What’s The Application?:
Imagine waking up at 5:40 in the morning after a night of studying Process Control, getting no more than six hours of sleep. I know, the first thing you want to do at this moment is get out of bed for a refreshing three or four mile swim. Sounds like heaven, doesn’t it? But alas the pool is chilly, and your morning, as well as the remainder of the day, is ruined. You contemplate your existence. You pee in the pool for the blissful but fleeting moment of warmth your own urine surrounds you with. But just as quickly as it came, it is gone. You are left in the cold yet again.
No more! Using a process control system, we can fix this vexing issue. Firstly, the volume of the pool is currently controlled completely manually. When an employee sees that the water level is low, they turn a valve allowing city water to fill the pool until they feel that the volume is at the right level, after which they close the valve. Additionally, the temperature is controlled by one non-variable heater. This means that when the volume is adjusted, the heat is not, and the pool is frigid until the steady state temperature is reached after a somewhat long time period.
How do we fix this? With SCIENCE!!
The volume (of the) solution (CHEM PUNS)
Our solution involves the use of a feedback control system when it comes to the volume of the pool. The water level of the pool, a function of volume, will be defined as our controlled variable. This is easy enough, simply use a water level sensor. These can be purchased online for very reasonable prices, and they are able to measure the water level when compared to a set point. This setpoint will be the desired water level and will change due to disturbances in evaporation rates or leaks in the pool. The water level sensor will then send a signal to a controller that will interpret the received data. When there is a negative disturbance in the water level, meaning that the water level is too low, this controller will cause a water valve to turn open. This will subsequently allow city water to enter the pool in an amount that will bring the water level back to the set point.
The set point will be fairly easy to determine. The water level should be measured in the diving well portion of the pool, as it receives far less human-related disturbance than the lap swimming portion. The diving well is supposed to be kept at a specific depth of about 13 feet deep. This is 3.96 meters in SI units, but we’re going to stick with feet because FREEDOM. It is then easily determined that the set point should be at 13 feet from the bottom of the diving well.
Also, it is important that the system only responds to negative disturbances. This is due to the fact that when the pool is in use, there will likely be a positive disturbance due to the added volume of bodies. If the system responds to this by draining water, it will constantly have to adjust as swimmers exit and enter the pool. This would be inefficient and extremely wasteful.
It’s getting hot in here, so take off your speedo (maybe not)
The temperature control system for the pool will also be controlled by a feedback control system. The controlled variable will be the temperature of the pool. The water temperature can be disturbed by changes in the air temperature and, more consequentially, the addition of city water from the volume control. Due to thermostats already in the Lafayette College pool, a disturbance in the air temperature will not necessarily have to be accounted for. Therefore, the only variable that can cause significant disturbance is the temperature and volume of city water added to the pool. In order to counteract this, a variable heater will be used to add some amount of heat ,Q, to the circulating water. This Q value will be our manipulated variable. Since there is already a circulation of water taken out of the pool to be filtered, the variable heater will work upon this filtered stream. In this way, there is no need to add another pathway for the heater, it is simply applied to the newly filtered water. The set point for this control system will be 77-82 degrees Fahrenheit (Freedom returns yet again), as dictated by FINA regulations for competition swimming pools.
Did he provide a diagram? He did!
What if I’m A feedforward kind of guy (or girl, science never discriminates)?
If you are looking at that diagram above and saying to yourself, I love this idea but gosh darn it I just hate feedback systems, then too bad, live with it. Just kidding! While feedback systems are great, it is true that sometimes they can have their drawbacks because (in this instance specifically) a temperature cannot be measured until after heat has been put in. Therefore, this could possibly result in a slower pool heat up time if we don’t know exactly how much heat to put in for a specific change in water temperature. Luckily there are two alternative options that can be used if one so pleases, however they keep the use of a feedback system for water level control. Number one, change our temperature control system to a feedforward one. In this instance, we would propose that the temperature of the incoming main city water would first be measured, and then heated to the desired temperature before it is deposited into the pool. Unfortunately this system has a number of potential drawbacks, as it would be unable to account for possible disturbance factors such as a slight air temperature change, and the inability to change the water temperature once the pool has been completely filled. That’s where a dual feedforward and feedback system comes into play. This proposed idea would both heat the main line city water to the desired temperature before depositing it into the pool (feedforward), and also use the feedback system noted earlier. So the entire process would sound like this. A sensor reads the level of the pool. If the sensor gives a reading below the setpoint, a controller will act to open a valve to deposit water into the pool. A temperature sensor will then measure the temperature of this incoming water and if it is below the setpoint, it will be circulated through a heater (inputting Q) until it is at the desired water temperature (77-82 degrees fahrenheit). This water will then be deposited into the pool. Once the pool is no longer being refilled a second temperature sensor will measure the pool water temperature, and if it is below the set point, a certain amount of heat (Q) will be added to the water when it is being passed through the circulating filter. However, while this option ensures the quickest heating times, it potentially uses both more money and more resources than our earlier proposed idea because two heaters are being used instead of one.
Okay then, how will this save the school money (and help the environment)?
As of right now the city of Easton charges $3.80 d per 100 cubic meters of water (plus a service charge), 5.61¢/kWh for electricity, and approximately $10.12 per thousand cubic feet for natural gas (state of PA). These values may seem small, but added up over a year can become exceedingly high. Every single time that the pool is filled too high precious money is wasted on the excess city water being pumped in. The water level control system easily fixes this problem by adding the actual needed amount of water to the pool. The temperature control system would help save money too. By signaling the heater to heat the pool precisely based on the temperature and known volume of the pool, no money would be wasted on natural gas or electricity for the heating system. These savings could then be used to better benefit the students of Lafayette in ways such as improving housing and academic buildings, or building a completely reasonable 10 million dollar glass elevator. Not only will this system make heating and controlling the pool level more cost efficient, but it will also decrease the pool’s impact on the environment. By using less water, electricity, and natural gas, valuable resources will be conserved and the amount of CO2 emitted by using excess natural gas will potentially decrease.
So, Can it be implemented in other areas?
Yes! This control system is not applicable to Lafayette’s pool only! The set point for volume will have to be altered for other pools, as not all pools will have a regular level of 13 feet. The temperature control system, however, can be applied to any pool to regulate temperature without any changes required. As such, our system, while specified for the Weinstein Natatorium, can be applied, with only minor changes, to any pool in the world.
- “FR 2 SWIMMING POOLS.” FR 2 SWIMMING POOLS | fina.org – Official FINA website. N.p., n.d. Web. 24 Mar. 2017.
- “AVERAGE PRICE OF NATURAL GAS (per 1,000 Cubic Feet).” Just The Facts – Average Price of Natural Gas – The Public Policy Institute. N.p., n.d. Web. 23 Mar. 2017.