

14102015, 17:33

#196

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Join Date: Apr 2012
Posts: 545

Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by sailorchic34
Dude, That is how engineers calculate it day in and day out. It's how the PE's taught me ages ago. It's also on the PE exam. So it's not just me. It's a whole lotta Professional engineers out there using it the exact same way I am.
The equations BTUH= Q*500*delta T. This really is an mechanical engineering formula.
Q = flow per GPM
500 = 8.33 pounds x60 minutes per gallon flowing. that is per GPM
Delta T is the temperature delta that is the difference between starting and ending temperature.
That equation is used in sizing heat exchangers...

I have the need for a heat exchanger that can cool a 50 percent wet vapor (saturated conditions) at 5 psig to 10 degrees subcooled at 24 inches of vacuum to allow efficient pumping without condensate pump cavitation, and the wet vapor flow rate is 4000 lbm per hour.
Assume a cylindrical, single pass heat exchanger with monel tubes 1/2 inch ID and 0.0625 tube wall thickness, and a worst case injection temp of 90F of the cooling water to the heat exchanger. Limit the diameter of the tube bundle of the heat exchanger to one meter. Select an induction motor driven cooling water pump capable of delivering 200 gpm.
How long will the heat exchanger need to be?
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14102015, 17:42

#197

Senior Cruiser
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
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Posts: 4,247

Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by StuM
When she said 50,000 MBH per minute, which part do we ignore. The "per minute" or the "H"? Did our "engineer" mean that we need to generate 5,000,000 BTU per minute (MBTU/m) or 5,000,000 BTU per hour (MBH). One takes 60 times as much power as the other.

Oh wait, maybe she meant we will be consuming 5,000 mBTU Hours every minute. In that case we need to generate 300,000,000 BTUs per minute
So which is our requirement:
300,000,000 BTU per minute or
5,000,000 BTU per minute or
83,333 BTU per minute?
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14102015, 17:49

#198

Registered User
Join Date: Apr 2012
Posts: 545

Re: Amps v Amp hours v Amps/Hr
SC...
Again, I commend you for engineering skills. However, there are gaps in your knowledge and abilities (as you admit) perhaps because of the manner in which you gained those skills.
If you were to bridge the gap that is so evident here...a lack of understanding of dimensional analysis...your other skills would improve immeasurably as soon as you did. I submit that there is no other single thing you could do to improve your understanding of science and engineering that would give such rewards. Somehow, you need to rethink what you know about the subject and start fresh on understanding this.
Think of this whole thread as an intervention...you are surrounded by those who would like you to master this topic and have tried to help you do it, and apparently failed.
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14102015, 17:55

#199

Senior Cruiser
Join Date: Nov 2013
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Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by sailorchic34
Let me rephrase that. A 5 gallon bucket would hold the flow rate of 5gpm after 1 minute of duration.
Better?

No. you can't fit a "flow rate" into a volume. As I have said several times, please try to grasp the fundamental difference between "quantity" and "rate"
It is the crux of this issue.
Try "a 5 gallon bucket would hold the total output of a flow rate of 5gpm after 1 minute".
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14102015, 17:59

#200

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Join Date: Oct 2011
Boat: Valiant 42
Posts: 3,016

Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by sailorchic34
Let me rephrase that. A 5 gallon bucket would hold the flow rate of 5gpm after 1 minute of duration.
Better?

No, it would hold 5 gallons after 1 minute. (The "of duration" is redundant because minute is a unit of duration.)
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14102015, 18:03

#201

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Join Date: Oct 2011
Boat: Valiant 42
Posts: 3,016

Re: Amps v Amp hours v Amps/Hr
SC..
I agree with previous poster who recommended a detailed understanding of units. All engineering equations must balance the units so the net units on the right match the left. If you know the units of every number or variable in the equation you can easily tell if the equation makes sense or not by ignoring the numbers and just check if the units balance (same on both sides). If the units don't match between left and right then the numbers don't matter because the equation is just gibberish. If the units balance then all you have to do is check whether the numbers are correct for the units they represent. If they are then you know the equation is correct. Balancing the units does not require any memorization of formulas. You can do it just by looking at the equation.
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14102015, 18:06

#202

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Join Date: May 2011
Location: On boat, currently Sea of Cortez
Boat: Celestial 48
Posts: 725

Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by sailorchic34
I actually understand the difference between quantity and rate. You can have 100 gallons and 100 GPM for 5 minutes. To different things, I get that. Yet just saying 100 gpm implies a rate duration per hour and a volume of flow in gallons. If I have 100GPM flowing I can calculate weight, mass, Btu's in minutes or hours, etc.

OK, late to this discussion, but interesting (have not read every post however).
but:
100 GPM does not imply a rate duration per hour. It is a rate of volume flow per minute.
It does not imply a volume of flow in gallons (sic). "volume of flow" doesn't even make sense. Should be rate of flow in GHP (e.g., your 100 GPM), or volume in gallons (which again can't be "implied" from 100 GPM).
If you have 100 GPM of flow you could calculate the flow rates in weight/time or mass/time (e.g., pounds per minute) but can not calculate the weight or mass as no time period has be defined to apply to the rates (you would need "100 GPM flow for 1 hour" as example).
Just given 100 GPM (and other data such as delta temp, densities etc) you could calculate BTU/hr but not BTUs.
On the other hand, if it was given "100 GPM for 1 hour" you could figure your other values. But this has to be stated, no way does "100 GPM" imply the "for 1 hour" part.
Quote:
Originally Posted by sailorchic34
of what I do engineering wise just uses GPM with no extra time units, such as GPM/minute or GPM/Hour. As most of what I do is GPM for 24/7...

Well good, as GPM per unit time (GPM/minute or GPM/hour) is rate of acceleration of fluid measurement, and unlikely to be used much in HVAC engineering. But "extra time units" when applied as factors you use all the time to calculate your volumes. For example the size of a storage tank with capacity to supply a 100 GPM pump with water for 2 hours would be 100 GPM x 2 hour x 60 min/hr = 12000 gallons.
I am sure in your actual engineering work you must be handling flow and volume correctly or projects would have ended badly, but your explanations fall far short of being accurate.
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14102015, 18:11

#203

Certifiable Refitter/Senior Wannbe
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 6,324

Re: Amps v Amp hours v Amps/Hr
Gosh, I leave the keyboard for a couple of hours and the thread moves on for 2 pages without SC commenting on the maths back in post 167. Perhaps she has been too busy with everyone else but nevertheless I feel miffed (OK, I don't really feel miffed).
So direction question for SC, whose maths are right back in post 167  yours or mine?
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All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence



14102015, 18:12

#204

Moderator
Join Date: Feb 2011
Location: SF Bay Area
Boat: Islander 34
Posts: 4,300

Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by fryewe
I have the need for a heat exchanger that can cool a 50 percent wet vapor (saturated conditions) at 5 psig to 10 degrees subcooled at 24 inches of vacuum to allow efficient pumping without condensate pump cavitation, and the wet vapor flow rate is 4000 lbm per hour.
Assume a cylindrical, single pass heat exchanger with monel tubes 1/2 inch ID and 0.0625 tube wall thickness, and a worst case injection temp of 90F of the cooling water to the heat exchanger. Limit the diameter of the tube bundle of the heat exchanger to one meter. Select an induction motor driven cooling water pump capable of delivering 200 gpm.
How long will the heat exchanger need to be?

Ah Steam condenser, so cool, er hot. I have to say, I've not sized a steam condenser with a vacuum pump. That's more power plant design, something I'm rather weak in. It's way too late in the day for this.
Sorry my comment was related to hydronic heat exchangers and then would only get the flow rate in GPM required through the heat exchanger in a single phase fluid.
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14102015, 18:19

#205

Senior Cruiser
Join Date: Nov 2013
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Re: Amps v Amp hours v Amps/Hr
""Can we now agree that GPH is not a volume? "
Quote:
Originally Posted by sailorchic34
No it is a two dimensional unit with one dimension being volume and other unit being time.
I do not need to calculate it an pounds per minute. Simple pounds works quite well. Its flow in GPM * weight X delta T Not weight/minute.

I'm not sure what that answer means. Once again your additional "wriggles" have confused rather than clarified the issue.
When you say No, do you mean:
"No, GPH is not a volume, so Yes we can agree." ( IOW a simple Yes)
or
"No, we cannot agree that GPH is not a volume, I still believe it is". (IOW, a simple 'No")
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14102015, 18:39

#206

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Join Date: Feb 2011
Location: SF Bay Area
Boat: Islander 34
Posts: 4,300

Re: Amps v Amp hours v Amps/Hr
Guys it's been fun, but I was pooped when I got up this morning and I'm really pooped now.
I'm an old woman who will be 60 years old in a rather short time. There is something about teaching an old dog new tricks. While my knowledge may be lacking, it is enough to get by with doing building hydronic systems etc. I will admit I am weak in theory, yet in practical engineering I'm still pretty sharp. As to balancing equations, I have computer software that I wrote 25 years ago that does that for me. After teaching myself C++.
Ya'll have fun, I'm going to make a big bucket of popcorn and read a book.
Ta
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14102015, 21:55

#207

Registered User
Join Date: Apr 2013
Posts: 3,722

Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by sailorchic34
OK guys, lets try it this way. I have a big pot with 10 gallons of water in it. If I pour it out so that after exactly a minute, the pot is empty of water, how much water did I pour out.

This is a word problem where the time is irrelevant. The only important items to know are the original volume was 10 gal and the final volume was 0 gal. How long it took makes no difference in finding the answer.
Most of your examples make the inverse mistake. Just because you know the flow rate, you don't know the volume...you then insert and assumed duration that was not part of the original question.
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14102015, 22:03

#208

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Join Date: Apr 2013
Posts: 3,722

Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by sailorchic34
I agree with this completely, it's flow volume per time.
So guys, In another thread today I was talking about heating water. So if we have a flow rate of 10 GPM. The water is at 40 degrees. I want to heat that to 100 degrees F, How many BTU's do I need to heat that 10GPM.
For the answer we convert 10 gpm to pounds. 1 gallon is 8.33 pounds at stp. So 10 GPM would be 83.3 pounds. We then multiply that by the delta of the temperature, 10040 for 60 degrees. 83.3 pounds*60 (F. delta T) gives an input of 4,998 btu/minute. Times 60 gives 299,880 btu's/hr. So 299MBH output will heat 10 GPM or 600 gallons in one hour 60 degrees F. This is a basic formula used in mechanical engineering around the world. Though some might use Cubic liters per minute.
Where did the pounds come from if 10 GPM is not also a unit of volume.

Since you don't know the volume, you can can't determine the BTU requirement. If you go back your final answer doesn't list the number of BTU required.
You made two key mistakes.
 If you don't know the volume you can't calculate how many lbs of water are being heated.
 At the end, you failed to provide how many BTU's are required.
What I think you were trying to explain but don't grasp is you can convert gal/min to lb/min factor in the temperature delta and get BTU/min. But until you know how long the flow lasted, you can't calculate the BTU requirement.
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14102015, 22:08

#209

Certifiable Refitter/Senior Wannbe
Join Date: Jan 2008
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Boat: Van DeStat Super Dogger 31'
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Re: Amps v Amp hours v Amps/Hr
Quote:
Originally Posted by sailorchic34
……...
I'm an old woman who will be 60 years old in a rather short time. There is something about teaching an old dog new tricks……....
Ya'll have fun, I'm going to make a big bucket of popcorn and read a book.
Ta

I must be a disagreeable mode today 'cause I reckon you are wrong again
59 going on 60 is not old; at least not in my universe
Enjoy your popcorn
And happy birthday  soon
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All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence



14102015, 22:20

#210

Registered User
Join Date: Apr 2013
Posts: 3,722

Re: Amps v Amp hours v Amps/Hr
[QUOTE=sailorchic34;1937538
I do not need a duration to calculate a BTU per GPM. Quite simple
its BTU=GPM* 8.33 pounds per gallon * delta T. You don't use pounds per minute and I've never heard an engineer describe it as pounds per minute. Not even once.
[/QUOTE]
So you have two hoses flowing at 10GPM. Each hose requires the same BTU to heat water (assuming the same starting and ending temperatures)?
Of course not. One hose might be running for 1 minute and require a relatively small number of BTU. Another might run for a 100minutes and thus require 100 times the BTU.
If you've never heard of lb/min (or an equivilent mass/time) then you haven't done the calculations.
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