Test question from today:

Eric_32

Took the Pipefitter license test today for RI, and got this question that had me thinking for while:



<strong>Is it more efficient to heat hot water using a steam boiler or hot water boiler?</strong>



Efficiencies and sizes were not given, so I am asuming both boilers would be the exact same brand and size, similar except for the duration they heat the water, hot water to 180°, and steam off the pressuretrol.



I didn't know if this references potable water as in a tankless coil, or a hot water zone which could be added to a steam boiler just like a water filled boiler.

I put they would be similar in efficiency as my answer thinking the steam boiler and water boiler could both be heated by the same model burner and would heat the water at the same rate getting water to the same temperature the same time, all happening before water got to boil.

Anyone know this and would like to share their answer?

thanks!!

 

icesailor

Answer?

I would say that there are more than one "right" answer.

IMO, if it is a hot water boiler is a hot water boiler, and you raise the water temperature to a point where you can make hot water but not steam, the hot water is more efficient. But if it is a steam boiler, and you are using s steam coil, and you have to run steam through it to make the hot water, the hot water boiler would definitely be more efficient.

I'm running out of battery power. I'll have to see the answer later.

RobG

Tough Question

That question sucks."Is it more efficient to heat hot water using a steam boiler or hot water boiler? "

1. Why are we trying to heat "hot" water? Are we trying to make it hotter? Are we trying to make steam?

2. If we are talking apple to apple boilers, they will both run at the same efficiency until the hot water boiler shuts of at 211 degrees and the steam boiler runs on. That would make the hot water boiler more efficient as it would use less fuel?

3. If we are talking a mod con vs. cast iron steam its a no brainer.

4. Are they talking domestic through a coil?

I'm getting overwhelmed at all the different possibilities of such an open ended question. Lets us know the answer and the reasoning if you ever find out. I'm putting my money on the hot water boiler just because the schools don't push steam anymore.

Rob

Paul48

Given the framework

A BTU is a BTU.

Eric_32

Asked my co workers this....

I asked my co workers this and one's response was:



Steam boiler would be as it is at 0 PSI and the water boiler is at say 12-15 PSI. As pressure increases so does boiling point, so he feels that formula is the same as water is heated up as well, which I don't actually know if that is true. (Does water under pressure take longer to reach a certain temp than at atmospheric pressure)?



Another mentioned boiler for boiler, side by side, the identical steam boiler would hold less water that the water filled boiler, so it would heat faster, therefore being more efficient.



Well I will find out in a couple of days, if I pass, I may never know. If if fail they will give me the wrong ones I think.

SWEI

translation

It's a bad question.



They're unfortunately common on licensing exams...

Brian Chilton_2

Hot water, because

the steam system requires the btu's to heat the water and the additional energy to change water to steam.  A phase change takes energy.

So it must be more efficient to just heat the water.



Think of it this way, if you built a perfectly insulated box around both systems, the hot water would be perfectly efficient to move the energy created by the boiler to the hot water tank.  But the steam system would lose the energy until it was released when the steam condensed somewhere in the steam pipes.

Paul48

The Question

"Is it more efficient to heat HOT WATER using a steam boiler or hot water boiler"?

Brian Chilton_2

how do you make hot water from a steam boiler

without making steam?

Brian Chilton_2

heat of vaporization

Heat of Vaporization







The energy required to change a gram of a liquid into the gaseous state at the boiling point

is called the "heat of vaporization". This energy breaks down the

intermolecular attractive forces, and also must provide the energy

necessary to expand the gas (the PDV work). For an ideal gas , there is no longer any potential energy associated with intermolecular forces. So the internal energy is entirely in the molecular kinetic energy.



The final energy is depicted here as being in translational kinetic

energy, which is not strictly true. There is also some vibrational and

rotational energy.





A significant feature of the vaporization phase change of water is the large change in volume that accompanies it. A mole of water is 18 grams, and at STP

that mole would occupy 22.4 liters if vaporized into a gas. If the

change is from water to steam at 100°C, rather than 0°C, then by the ideal gas law that volume is increased by the ratio of the absolute temperatures,

373K/273K, to 30.6 liters. Comparing that to the volume of the liquid

water, the volume expands by a factor of 30600/18 = 1700 when vaporized

into steam at 100°C. This is a physical fact that firefighters know,

because the 1700-fold increase in volume when water is sprayed on a fire

or hot surface can be explosive and dangerous.

Gordy

Over thinking it

Maybe. If that is exactly how the question was worded the water to be heated is already hot.





Kinda like the question: If you dig a hole 6'x6'x6' how many cubic yards of material is in the hole? Answer: 0. There is nothing in the hole.





So I agree with Paul48





Gordy

Paul48

I'll answer with a ?

How do you make steam in a steam boiler without heating the water?

VictoriaEnergy

I'd say hot water

Assuming the hot water we are heating is DHW, then a further reasonable assumption is the load varries, so the boiler cycles.  Hot water would win.

Gordy

Not enough info given

To give an accurate, and complete answer. Like Paul said a Btu is a Btu.



The question does not indicate if the water is domestic, or boiler content water. Nore does it indicate what temp the water is, and to what temp the water will be heated to, or what pressure the water to be heated is.



All in all a Btu is a Btu. If boiler efficiency is identical then they should be the same.

Brian Chilton_2

I agree with Paul too

a btu is a btu. Putting it another way, energy is neither created nor destroyed.

The first law of thermodynamics:

First law of thermodynamics: Heat and work are forms of energy transfer. Energy is invariably conserved but the internal energy of a closed system changes as heat and work are transferred in or out of it. Equivalently, perpetual motion machines of the first kind are impossible.



So, for this question, which obviously leaves out details on systems, but only mentions water and steam as ways to heat water, they would be identically efficient until the steam system made steam. At that point any heat of vaporization became "friction" in the system and is energy transferred away from the "task at hand" of heating the hot water.







Asking the question another way, if they are equally efficient, where did the energy come from to vaporize the water to make steam?

Paul48

BTU

 A British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by one degree F.

 Now take an identical weight and temperature water in both types of boilers. It will take the same btus to raise the temperature of that water(and still have it be water), therefore it will have used the same amount of energy. The efficiency is the same.

Bob Vennerbeck

off topic - but I love VictoriaEnergy's signature line

Not a heating pro by any means - I come here to learn, and to share when I have experience that is relevant.

My day job is computer - electronics - video troubleshooter - and I agree wholeheartedly with your tag line:

As troubleshooters, we often find out that we've missed - wouldn't it be great to get feedback when we hit the bullseye?



Vbob