Temp and pressure question - Ron Jr.

Unknown

Ron,

There is a difference in how they will perform. The last article I remember seeing describing this was in the Technical section of an A.O. Smith catalog. The amount of recovery can change quite dramatically based on pressures.

I got involved in a situation several years ago when one of the local water utilities cranked up the street pressures to combat air pockets and a bacteria problem in their mains. We were getting boilers that would not reach high limit and indirects that would not recover. We were getting phantom pressure rise in the boilers because of the increased street pressures. The boiler PR valves, like most, were calibrated for a 50 psi to 12 psi reduction which changed dramically with the higher 100 psi incoming pressure. Had to temporarily isolate the boilers to keep the pressure down until Main PRV's could be installed. Incidently, we spoke with the water utility who denied running pressures that high. Later that month, however, they hung notices on every door of that development advising on the installation of a Main PRV! Hope this helps.

Glenn Stanton

Burnham Corp.

Unknown

Say you have 2 water heaters ,

Both are identical in every way except that one tank is in a house with 50 psi. street pressure , and one is in a house with 100 psi .

Will one tank heat faster or slower due just to the differences in pressure , and is it significant ? Thanks .

Robert O'Connor

We

Install pressure reducers to bring down the incoming pressure to about 60psi. This has solved not enough hot water complaints in indirect fired tanks.

I think it is significant . All tanks have first hour ratings etc. If you are exceeding your flow rating you will have "not enough hot water".

If you are doubling your pressure how much are you increasing your flow? 2x?

Regards,

Robert O'Connor

Bob Morrison_2

Trick question?

Ron,

There's no difference in the rate water would heat at each pressure. The effect of pressure on water's specific heat is negligible. Water at rest in the two tanks would recover equally quickly.

However, as Glenn points out, inlet pressure affects flow rate, and therefore, recovery rate through the heater.

Bob Morrison

Unknown

Thank you Glenn

I just wanted some more facts to recite to customers who have high pressure in their houses , and how it can affect the heater , and boiler - besides reducing the performance , it will cause tank failure prematurely , and also have undue stress on all the piping . All the more reasoning to install whole house pressure reducers , when needed.

Robert O'Connor

Facts

The only they like is it will save them money. There is alot of water wasted when the pressure is over 60 psi.

Good point about stress on tanks.

Regards,

Robert O'Connor

Robert O'Connor

Facts

One fact they like is it will save them money. There is alot of water wasted when the pressure is over 60 psi.

Good point about stress on tanks.

Regards,

Robert O'Connor

Unknown

Aint no trick involved

But I was just thinking about it - Doesnt pressure have some sort of effect on how long it takes to heat water ? You said its negligible , but by how much ?

Say its 2 tanks . Both are full of water , but closed off , so theres no flow rates to worry about. Theyre both the same temp , but very different pressures . Will both tanks heat up at the same time ? This stuff goes into physics and science , which aint my strong points , so my honest to goodness question might look naive to others .

Unknown

Robert

Im sorry - it was a bad choice of words for the question . What I wanted to ask was - does pressure alone affect how fast water can be heated . Matter of fact , a customer asked the question just today - and I couldnt answer it . Thanks .

Unknown

Article

Ron,

I will see if I can come up with a copy of the A.O. Smith article on this topic. I wll let you know tomorrow or Thursday.

Glenn

Robert O'Connor

Physics aint my strong point either but

It takes btu's to heat water.

Water isn't compressible ( so you can't squeeze more in a tank).

Does the heat transfer to the water better at different peressures? This is your question? I think as a practical mattter no but ... maybe one of The Wall Wizards will enlighten us.

Regards,

Robert O'Connor

Unknown

Exactly

That was the question - you worded it perfect . Thanks Robert .

Unknown

Glenn

Once again , thanks for the help . I was just on a job where the relief on the Taco Dual feed and relief kept going off. The dual unit was installed a few months before . Without having a coil - the problem was either the dual unit or expansion tank . Being in a finished basement , I changed both .

Then I did a very strange thing , I read the instructions that come with the feeder . It said that 100 psi was the maxinmum the feeder can handle . So I broke out my pressure gauge and hooked it to the outside hose . On a very hot summer day - presumably with most of the town watering lawns - the pressure was 105 psi . I called in and said they could use a pressure reducer for the house before theres a flood in the basement .

Boilerpro

Hey Glenn

You still doing some training in the Chicago area this summer? Just signed up for Engler's Seminar put on by Ron Beck Aug 15 in Hillside.....combined high/low temp systems and steam to hot water conversions. Like to meet up with you some time.

Boilerpro

Online Sheriff \"Murph\"

hey glenn

could i have that in writing please??

Unknown

I will be

out there on September 4th in Oak Brook doing a full day of Gas Troubleshooting and Modular Heating Systems and in Indy the next day. These will be sponsored by G.W. Berkheimer. We brought Ron on board last month and he will be handling most of my Midwest and Western Region schedule while I concentrate my efforts in the Northeastern States. Ron is a great guy and is very knowledgable. I am sure you will enjoy the day very much! I will also be back out on October 29th in Oak Brook for an all day steam seminar. Thanks for asking!

Glenn

Unknown

Water Heaters

It takes 1 BTu to raise the temperature of 1 lb of water 1 degree (F). A gallon of water is 8.33 lbs on say a 30 gallon water heater that is 240 lbs. A 30 gallon water heater will recover approx 30 gallons of water per hour 90 degree rise. Computed as follows:

8.336 lbs x 30 gallons = 250.8 lbs

62 degrees to 140 degrees is roughly 80 degrees

80 x 250 lbs = 20,000 btus minimum input for a 30 gallon water heater

Then divide the BTUH required to heat water by .70 (heater efficiency) to determine heater input. (Some models are 75% efficient)

20,825 divided by .70 = 29,750 actual input to heater.

That is the basic principle of water heaters.

When we talk about pressure and temperature water under 50 PSI can be superheated to 290 degrees without it boiling. If there is a rupture or the systems is opened high temperature high pressure superheated water flashes into steam and we have instant explosive force of the excess energy stored in the superheated water. Steam expands 1700 times more than water.

The water at rest at any given pressure will simply absorb BTU's. A 40 gallon water heater with 80 psi line pressure will build to a pressure of 145 PSI with a temperature increase of 16 degrees (F). As the temperature increses, the maximum working pressure for which the heater was designed can be exceeded (150 PSI).

The answer to the question is that increased pressure should have no discernible effect upon temperature or time to heat when water is at rest. When water begins to flow it will depend on GPM and pressure and the ability of water to absorb heat at different flow rates.

Brian (Tankless) Wood

I agree totally and absolutely

with Sir Timmy.

A btu is a btu, a pound is a pound, and a degree is a degree, period.

I'm sure a storm is brewing though!!!!

Regards from Brian (basic physics in swampland) Wood.

Bob Morrison_2

theory says: no difference for liquids; gases are otherwise

Ron,

The theory is that substances which are mathematically modeled as incompressable (solids and liquids) have specific heat values that do not depend on pressure. Compressible substances (gases) are a different story: their specific heat does depend on pressure.

I know what you mean: it seems like the pressure would help the heat transfer. And these types of questions are interesting. I defer to the physicists to talk about the limits to the models. I'm just retelling what's in the thermodynamics books. I'll post or send you a reference if you want.

Bob Morrison

Jacob Myron

Capacities of water heaters and water pressure.

Write this quick formula Down it will help you figure out a lot of problems.

10 times the diam sqared times the sqare root of the pressure.

Lets say the pipe to the heater is 1" and the pressure is 50 psig.

The discharge of a bath tub faucet is from a 1/2" pipe.

.5 x .5 x 10 = 2.5

2.5 x the square root of the prssure is 17.6 gallons per minute.

At 100 PSIG it become 2.5 time 10 or 25 GPM.

We know the bath tub faucet has a restrictor in it but you can see that the gpm flow rate increases as the pressure increases.

Doubling the pressure increases the out flow almost 1.5 times.


Jake

Drod

I remember my first chemistry experiment at Joliet Catholic High

School in 1965. We had a beaker of water, and heated it to boiling. We removed it from the heat source, it stopped boiling. We corked the top. Then, plunged the beaker into cold water and it starting boiling again. While I remember the experiment, I don't remember if it had anything to do with pressure...maybe it was vacuum.

Unknown

Pressure and Temperature

Ron,

Apparently I did not get the entire point from your original post. The comments I made last night in my post to you were for a situation that was kinetic and not static. Robert and Jacob are correct in their posts that when static (no flow) the heaters will have no discernable difference in their ability to transfer btu's. The article I was referring to was for kinetic or flowing recovery differences.

The jobsite problems I referred to were caused by excessive pressure coming into the houses with poor recovery of the heaters while being used. The secondary problem of phantom pressure was causing the boilers to weep past the relief valves and in some cases, temporarily flash for a second or two. I will still get a copy of the article and send it to you. Sorry for the misread on my part! Guess I'll have to start going to bed earlier from now on! I will be away until Monday night to attend my daughter's wedding in Canada. Talk to you next week when I return.

Glenn

Unknown

Flow Chart

Ron,

Here is a scan of a flow chart showing the flow of different sizes of pipe at differing pressures. This is out of the current A.O. Smith Catalog. I am still searching for the article out of the older catalog discussing this subject at length. Hope this helps.

Glenn

Unknown

Flow Chart

Ron,

Here is a scan of a chart conveying flow rates of various sizes of pipes at differing pressures. This is from the current A.O. Smith catalog. I will also fing the article talking about this at length. It is in one of my older catalogs that is still packed away somewhere. Hope this helps.

Glenn