Most of us these days have experienced the communication method known as texting. If you have not texted before, I commend you for getting by this long in life using more traditional methods of communication such as speaking.
One of the skills used in texting requires knowledge and use of abbreviations. Some of the more common ones are LOL for “laughing out loud” or BRB for “be right back.” Engineers and scientists have used abbreviations for centuries to tell a story. Examples include gpm to describe the amount of fluid moving from one location to another, or psi to describe the amount of pounds force applied to an area of 1 square inch.
One of the most interesting abbreviations used in our field is Btu. Btu is short for British Thermal Unit. The story Btu tells is of “the heat required to raise one gallon of water, one degree Fahrenheit.” The Btu is commonly denoted as “Btu/h,” which is the amount of thermal units per hour. Along the way, the “h” got dropped so that when we use Btu, we are usually referring to the amount of heat conveyed in an hour. This is another characteristic of communicating with abbreviations; they become so commonplace that the abbreviations themselves get abbreviated.
What other unit describes so much in our field as the Btu? From natural gas, to insulation loss, to heat content in water and air; both sensible and latent, Btu connects our trades together, and it would be wise for us to become familiar with it. There are two applications I like to apply my knowledge of the energy-filled Btu to. The first involves heat input and the second is heat output.
Sizing a water heater involves adding heat to water in order to raise the temperature of the water. In the Northeast, we use a temperature rise of 100° F. If we calculate the amount of water that needs to be heated, we can convert the gallons to pounds and find out how many Btu are required. For many jobs I’ve been working on, the emergency shower has the highest demand. Right off the bat, if I estimate 10 gpm of hot water is needed for the shower, I can convert this to 83.3 pounds of water per minute, which then becomes 5,000 gallons in an hour. At a 100° rise, this requires 500,000 Btu in 1 hour. Below is unit conversion that shows my reasoning.
The calculation is a bit on the conservative side, but it can give you an idea of how we can calculate the Btu input of a water heater required and the unit conversions that lead to the story told by the “Btu.” This calculation would be for an instantaneous heater and not include any storage. This is where your engineering judgement comes in: How much storage would you provide to offset the instantaneous heating load? Here’s another twist: What if the owner tells you all they have is electricity? Another very important conversion to have in your tool belt is to go from Btu/h to kw. There are 3,413 Btu in a kw, so the amount of kw needed to operate this emergency shower could be as high as:
I’m not going to even try to tell you what size breaker that requires. All I know is that turns out to be 146 toasters and I can usually pop a breaker with a 1-kw toaster that is sitting on my kitchen counter just by using the microwave at the same time.
Another design problem I come across as a plumbing engineer is one of heat loss through insulation on domestic hot water piping. Maintaining domestic hot water at a satisfactory temperature is of critical importance for various reasons. Remember that emergency shower in the example above? Some jurisdictions require the hot water be recirculated to within 5 feet of the shower’s mixing valve. It’s up to plumbing engineers to make sure the hot-water recirculation pump is sized so that all the Btu that are lost through the insulation get put back in. Using a spreadsheet I have, I did a quick theoretical calculation that shows heat loss through a hot-water distribution system composed of the following pipe sizes (shown above).
Now the design challenge becomes, how do we replace 31,500 Btu/h in 84 gallons of water that are circulating through 1,700 feet of insulated copper piping. One important consideration for this design problem is: What is the acceptable temperature drop before the recirculation pump turns on? The way I look at it is 1 gallon of water, being 8.33 pounds, and leaving the mixing valve at 130°, will add around 8.33 lb. x 10°F = 83.3 Btu to a system that has cooled down to 120° in an hour. Now, we should ask ourselves how many pounds of 130° water do we need to circulate in order to replace 31,500 Btu/h? To me, the answer would be:
This is one way to size a hot-water recirculation pump to replace heat loss through piping insulation. I welcome your thoughts about how you would perform the same design challenge.
Engineering units remind me of “texting” because they tell a story in short format. It would sound pretty odd if we called our local water heater representative and said, “I’d like a water heater that can heat 41,650 pounds of water 100 degrees over the period of one hour, please.”
They would either think you were a really great engineer or maybe that you just landed on Earth from another planet. So, you can see how engineering units help us in our design efforts just like abbreviations help convey information when we type away on our little devices.
We should try to remember abbreviations are great for conveying information. When it comes to texting an “LOL” though, I’d much rather hear a friend’s laugh over the phone or across the lunch table “IRL.”