The Conservation of Energy
What is the Conservation of Energy?
In our last segment, we introduced Hydrokinetics.
Today, we’re going to look at the total energy of the system in the context of the Conservation of Energy. We’ll then introduce a few examples to explore this concept in our world of fire suppression.
This module covers an important principle; the Conservation of Energy. For this discussion, we will rely on both physics and fluid dynamics.
CONSERVATION OF ENERGY
So, what is the Conservation of Energy?
This is an important idea. This principle says that within a fixed system, the total energy will remain constant, provided that we don’t introduce or remove energy from that system.
So, Total Energy of a closed system is only the sum of potential energy and kinetic energy.
ETOTAL = EPOTENTIAL + EKINETIC
Think about that for a moment.
Since Total Energy is constant, a change in potential energy must be matched by an equal (and opposite) change in kinetic energy.
What can create that change?
if the closed system is static there is no kinetic energy only potential stored energy – no motion of the fluid.
If the fluid is in motion, a portion of the potential energy creates fluid motion -- this is kinetic energy.
Interesting but what does this have to do with fire suppression?
EXAMPLE: PIPE TRANSITION
Fortunately, a large number of practical engineering flow problems involving water can be modeled as one-dimensional, steady flow problems. In particular, this applies to many pipeline flows.
When we look at any two points in the system; the total energy at each of those two points is going to be the same.
Here’s an example situation, using a 4-inch pipe on the left that is connected to a 2-inch pipe on the right using a concentric, grooved reducer. Let’s imagine that the fluid flow during this transition is smooth and laminar.
Here’s an example situation:
We have a closed system with an incompressible fluid, like water. We have two points in the system, Point 1 and Point 2. Point 1 has a larger-diameter round pipe. Downstream we have a concentric reducer that takes the flow to a smaller-diameter, round pipe, at Point 2.
Water is flowing at both points in the system, to the right.
Our principal concept today is that the total energy of a closed system does not change between these two points, yet it is the same. It remains constant.
Now, there are different forms of energy.
Total Energy is the sum of Potential Energy and Kinetic Energy. We brought that up earlier.
EXAMPLE: WATER SUPPLY
Let’s look at another situation in this context.
Here, we have a water tower on the left, which feeds a sprinkler system on the right.
What is happening here, in our context of Conservation of Energy?
The sprinkler system is actively flowing water. Water is moving at a relatively high velocity through the opening of the activated sprinklers.
The branch pipe serving the sprinklers has water moving at a relatively high velocity before the water reaches the sprinkler.
Now, going all the way back upstream, there is a large body of water at the top of the water column (at the water tower). Is this water in this big body moving? Well, it is, only very slowly. The water level is very, very slowly moving downwards. It’s actually moving so slowly that for calculation purposes, it’s velocity is essentially zero.
Where is the energy in this context? For a closed system, the total energy at two points in the system is the same.
Now on the left, at the water tower, we have a relatively high amount of potential energy. The water is stored at a higher level than the rest of the system, but is hardly moving at all. Here, the total energy is made up almost entirely of potential energy.
Now to the right, at the lower point in the system where we have water actively flowing, we have far less potential energy, but we have significant water movement. Here we have higher kinetic energy. The total energy in the system, between a point in the water tower and a point where the water is flowing at the sprinkler, is the same.
Now, just to expand on this a little, energy can be added into a closed system. How is that done? What if we added a fire pump in the middle of this system?
An electric fire pump, for instance, would take electrical energy and perform work into the system. It’s actually converting electrical energy into heat, sounds, and movement which increases the pressure into the system. If it is highly efficient, it’s converting electricity into mostly added water pressure in the system.
Can we account for the added work in the system? Well yes, we can. We’ll explore that with some numbers later.
EXAMPLE: SPRINKLER SYSTEM
Let’s look at one last suppression example.
Here is a wet pipe sprinkler system. On the left we have a sprinkler riser, which is fed from an underground water service. The riser has a backflow preventer, valves, flow switches, and a few other features.
It feeds a cross main, which runs perpendicular to branch lines. At the top of the cross main is a riser nipple that converts the feed main to the branch lines.
Finally, the branch lines bring water out to the sprinklers.
Let’s say we have two sprinklers that are activated. Did you know that in over 90% of fires, only two sprinklers are enough to suppress a fire until a fire department intervenes? That’s the most common result for a fire in a sprinklered building.
Let’s say two sprinklers have activated and are distributing water below.
Is the total energy of our system the same at the pressure gauge on the riser as it is on the branch line serving the activated sprinklers? There pressures are not the same – does that mean the total energy is not the same?
Remember the Conservation of Energy. The Total Energy in a closed system is the same at two points on the system. If our pipe was perfectly smooth and we had no losses due to friction, and we add no pressure along the way, then the Total Energy at each point in the system will be exactly the same.
That’ Conservation of Energy.
You’re gonna ask me: But Ed – don’t we live in the real world? How could this be helpful for us if our pipe isn’t perfectly smooth – and we don't have friction losses?
The Conservation of Energy lends us some helpful models where we can incorporate all of these things about our system and calculate answers to the important questions that we have.
How do we design a system so that we get the flow to a sprinkler that we need to suppress a fire?
What if we need more flow at a sprinkler?
What if my cross main jogs a whole bunch of times to avoid steel beams?
What if my roof deck is actually 25-feet instead of 15-feet?
Our knowledge and model around these important principles is what allows us to calculate those answers.
So, let’s recap, what the Conservation of Energy?
It’s a principle that states that the Total Energy of any two points in a closed system will be the same, as long as we are not imparting or extracting energy from the system.
In our next segment, we will introduce the Conservation of Matter, and then tie these two principles into our working model of fluids.
I’m Ed Henderson, this is MeyerFire University.
Leave a Reply.
Sentry Page Protection
Aaron Johnson, CFEI
Al Yakel, SET
Chris Campbell, PE
Chris Logan, CFPS, RSE
David Stacy, PE
Ed Henderson, PE
Joe Meyer, PE