Calculating BTU & Water Consumption for 50 kW/h Steam Turbine

[burnit]

Hi All,

For a small steam turbine to generate power at a rate of 50 kW/h.
How many BTU's of heat required per hour, also water consumption per hour, how do you work this out?

 

[russ_watters]

Well, the water consumption should be zero in a closed system.

To figure out the BTU input, you need either the catalog data from the turbine or if this is just a theoretical exercise, you need to pick points in a steam cycle and a turbine efficiency. For a short answer, steam cycle efficiencies can be around 40%, so that's 125 kW input, or 427 mbh.

 

[RonL]

Hi All,

For a small steam turbine to generate power at a rate of 50 kW/h.
How many BTU's of heat required per hour, also water consumption per hour, how do you work this out?

1 BHP (boiler horse power) is 33,472 BTU/hr
1 BPH is equal to 34.5 pounds of water/hr into steam @ 212 F degrees

1HP = 2545 btu/hr, or 44.2btu/min,

hope this helps

Maybe that looks better ?
Thanks Russ

RonL

 

[russ_watters]

Yikes, no, RonL. You're mixing up power and energy:
Units of power:
W, kW, HP, BTU (shorthand that really means BTU/H)

Units of energy:
BTU
kWh

There is no such thing as kW/hr, hp/min, W/min, etc. and all common energy numbers are in increments of hours. The RPM doesn't enter into a power or energy calculation, so it doesn't matter that it is in min. Amps (and watts) are power in increments of seconds. (C/s and J/s).

Ie, 50 kW is 50,000 W or 37 hp. All three of those are power and the full units are kJ/s, J/s, and ft-lb/sec, respectively. 1 hp is 746 W, not 746 "w/min", which doesn't exist.

Also, 1 BTU (energy) .293 kWh.

 

[burnit]

Thanks for the comments,

How are generators--alternators rated?

Are they rated per hour, say a 50kW generator as mentioned is this rated at a 50kWh generator.

It seems if we apply a 50kW load to it for 1hr it has produced a lot more than 50kW because it is drawing 50kW continuously?

Can someone explain this.

 

[RonL]

Russ, I know the 746/min was wrong, but can we use a real live situation and make use of time as if I'm setting up a power system for my home (in summer my bill goes crazy) this is a low month for me, the usage is 1438 KWH.

There are highs and lows, so I would need a method of storing power to handle the peak points of energy demand.

If steam is at hand, and a turbine spins at 36,000 rpm, a 10:1 reduction is needed to supply 60Hz power, the generator speed will be 3600 rpm.

Would it not be right to do this 1438 X 1000/28/24/60 and arrive at 35.664 watts per second (J/s)
The definition of a BTU is energy (watts) to raise 1 pound of water 1 degree F in 1 minute.

The amount of steam needed will be based on quantity and pressure to drive the turbine at 36,000 rpm and cover losses in the reduction unit and generator, while delivering a final wattage from the generator at 3,600 rpm.

Also needed, would be coverage for losses in storage and distribution of the power over the 28 days.

At one time in the past I had arrived at the conclusion that a 10KW generator head would be needed to cover through the hot summer days.

What did I miss, if anything ?

Thanks
RonL

 

[RonL]

Thanks for the comments,

How are generators--alternators rated?

Are they rated per hour, say a 50kW generator as mentioned is this rated at a 50kWh generator.

It seems if we apply a 50kW load to it for 1hr it has produced a lot more than 50kW because it is drawing 50kW continuously?

Can someone explain this.

Hi burnit,
Sorry if it looks like I hijacked your thread, just thought my example would fit in with your questions.

The belt driven generator unit I have, is rated at 10 KW (10,000 watts) @ 3600 rpm, however the specs caution against going beyond 9.5 KW (9,500 watts) for more than a few seconds, and a steady rating of 7.5 KW (7,500 watts) for continual output. The recomended power is around 15 hp.

In my Ford Ranger I have a 200 amp altenator, it is regulated to deliver 12 volts, and rectified to DC. At that power level it delivers 2.4KW (2,400 watts) but is generally considered by its amp output.

Both units are alternators that deliver 3 phase power, which is converted to single phase, the 10KW generator is designed to deliver both 120 volt AC, and 220 volt AC, while the automotive unit is regulated to 12 volts, and rectified to DC.


There are a lot of ways to mix and match power and generating units, but total wattage on a steady draw, will determine what your minimum power has to be.

Ron

 

[russ_watters]

How are generators--alternators rated?

Are they rated per hour, say a 50kW generator as mentioned is this rated at a 50kWh generator.

It seems if we apply a 50kW load to it for 1hr it has produced a lot more than 50kW because it is drawing 50kW continuously?

Can someone explain this.

Again, the difference is power vs energy. kW is power, kWh is energy. Power is a rate of energy production/consumption. All it means for your example is that 50 kWh is 50 kW for one hour. It's the similar to if you say your speed is 60 mph, for 1 hour, your distance is 60 miles.

Or, if you would prefer: 50 kW is 50,000 J/s. 50 kWh is 50,000 J/s/hr * 3600 s/hr = 180,000,000 J.

It could be that the fact that the time for the rate is included in the unit (W=J/s) is confusing you.

 

[russ_watters]

If steam is at hand, and a turbine spins at 36,000 rpm, a 10:1 reduction is needed to supply 60Hz power, the generator speed will be 3600 rpm.

Would it not be right to do this 1438 X 1000/28/24/60 and arrive at 35.664 watts per second (J/s)

I'm not sure what those numbers are supposed to represent, but again, there is no such thing as "watts per second". Watts already has seconds in it: 1 W = 1 J/s, not 1 W/s = J/s

The definition of a BTU is energy (watts) to raise 1 pound of water 1 degree F in 1 minute.

I don't know why you threw watts in there, but again, Watts is power, not energy. And being energy, a BTU has no time component: a BTU is the energy required to raise 1 lb of water by 1 F. Period.

 

[russ_watters]

You changed this post...

1 BHP (boiler horse power) is 33,472 BTU/hr
1 BPH is equal to 34.5 pounds of water/hr into steam @ 212 F degrees

1HP = 2545 btu/hr, or 44.2btu/min,

hope this helps

Maybe that looks better ?
Thanks Russ

RonL

That all looks fine except I don't know why you would ever use btu/min. But the numbers and units are otherwise correct.

 

[RonL]

You changed this post... That all looks fine except I don't know why you would ever use btu/min. But the numbers and units are otherwise correct.

A vivid imagination and a simple mind, I have to reference conversion tables, and have faith that most are correct. A ton of refridgeration doesn't mean much to me if spread over 24 hours, but I read that as a rule of thumb, one horsepower is required for each ton delivered in a system, and at a rate of 42.2 BTU per minute I am using 1 horsepower, or 746 watts to make that happen.
I know that a 3,700 watt heat element placed in a 50 gallon water heater takes a while to heat the water to a desired temperature, and that putting the same element in a properly designed container with a quart of liquid around it, things happen much faster.

Most everything I use is measured in volts, amps, watts, and revolutions per minute, also learning metric is coming slow for me, but I am making some progress, thanks to you and others on this forum.

Ron

 

[russ_watters]

A ton of refridgeration doesn't mean much to me if spread over 24 hours, but I read that as a rule of thumb, one horsepower is required for each ton delivered in a system, and at a rate of 42.2 BTU per minute I am using 1 horsepower, or 746 watts to make that happen.

You are still mixing power and energy. A Ton of refrigeration is power, not energy, so it is not and cannot be 12,000 BTU delivered in 24 hours, it is 12,000 BTU delivered in one hour. Also, I don't know where you got that rule of thumb, but for houshold a/c units, it's twice that. Your conversion is also wrong: 1 Ton of refrigeration is 12,000 BTU per hour, which would be 200 BTU per minute.

 

[RonL]

Guess I'll just stick to putting things togather and stop trying to explain what I think is happening.
On a positive note, it might serve to show anyone down in their spirits, to buckel up and see it through. (get that degree)

I do have two more 3.5 hp DC motors, and a hand full of 35 amp rectifiers on the way

Later
Ron