Efficiency Extra Edition
BY TINA TOBUREN, P.E.
Announcements
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Read & Comment
on Tina’s Blog: |
Next LM6000 Seminar
to be held in Palm Springs on
March 12 & 13, 2009 |
Calculating Heat Rate: Q&A
Even with today’s fuel prices dropping the way they are, calculating and
monitoring your heat rate is important. Changes in heat rate can indicate problems
with your unit – problems may include instrument calibration drift, gas
path fouling or foreign object damage (FOD).
Although, in most cases, performance losses due to FOD are noticeable
without getting out the calculator.
There are only three numbers that go into the heat rate calculation, so it
should be simple, right? But, when you
look at those three numbers a little closer, several questions can come up.
First, the calculation:
Heat
Rate = Fuel Flow * Fuel Heating Value / Power Output
The first question is: What are the engineering units on these
values?
In the US, Heat
Rate is most often shown in Btu/kWh. Fuel Flow can be in a number of different
units, the most common being KPPH (thousands of pounds per hour), PPS (pounds
per second) or SCFM (standard cubic feet per minute). Fuel Heating Value might be provided in
Btu/SCF or Btu/lb. Power Output is
nearly always in either kW or MW.
For places outside
the US, Heat Rate is most often shown in kJ/kWh. Fuel Flow might be reported in
m3/hr (standard cubic meters per hour) or kg/hr. Fuel Heating Value may be in GJ/kg, or
GJ/m3. Power Output is still in either
kW or MW.
As long as your fuel flow rate and fuel heating value are in compatible
units (both mass basis or both volume basis), your units should cancel
out. If not, you will need to know the
density of your fuel (kg/m3) in order to convert them to a common basis. To calculated the density of the fuel, you’ll
need to know the constituent analysis:
for natural gas, this would mean the volume percent of Methane, Ethane,
Propane, Hexane, etc. Note: Industry standards, such as ASME PTC-22,
provide guidance on converting the constituent analysis to a density (as well
as calculating the heating value).
The next question is: Where does the
fuel heating value come from? The
best answer for this question is to have your own gas chromatograph or heating
value lab on site. A more common source
of fuel heating value and constituent analysis is your fuel supplier. If the supplier cannot provide you the detail
you need for the time frame you need, or if there are mixing stations between
their reporting station and your unit, you may need to take your own fuel
samples and send them to a laboratory for analysis. You’ll need to determine the source of your
fuel heating value prior to calculating heat rate, just in case you do need to
take your own samples. Samples need to
be taken at an approved location (free from moisture or other ‘heavy’ particles
that are filtered out prior to combustion in the unit), and must be transported
in an approved container (an approved pressurized cylinder for natural gas) to
an appropriate lab.
Once you have your fuel heating value source, the sources for the other two
values must be found as well, but luckily, these two are normally easier to
identify. There are normally two choices for fuel and power output: At the unit, or at the plant boundary (i.e.
the billing meters). The heat rate you
need to determine will define which meters to use.
For a gas turbine unit heat rate,
you’ll want to record the fuel flow to the gas turbine at the meter closest to
the unit and the power output from the power meter on the gas turbine generator
– again, at the meter closest to the unit
For an overall plant or facility
heat rate, you’ll probably want to use the billing meters for both gas and
electricity – and this should therefore be a net heat rate for the facility
(after all auxiliary and house loads have been accounted for).
There is still the question of uncertainty, or: How
accurate do I know my calculated heat rate? For an ASME PTC level test, facility heat rate
should be known with an error band of less than 1.5% (including corrections to
reference conditions). When doing spot
checks for heat rate using permanently installed instrumentation and a fuel
suppler reported heating value, the uncertainty may be much higher – it all
depends on the sources of your information.
I hope this helps you on the way to calculating – and trending – your own
heat rates. Please contact me if you need any help in determining the best data
sources to use or in setting up worksheets for unit conversions and fuel
density calculations. I also have
add-ins for Excel which can automate a lot of the calculations for you (see
below for more information).
T2E3
Products & Services
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Analysis Tools |
Excel Workbooks, Macros & Add-Ins |
Training Seminars |
Compressor Efficiency Tracking Software |
Site Marketing and Dispatch Support |
T2E3 Add-ins for Excel
Streamline your analysis
spreadsheets by using functions from the T2E3 Automation Add-ins. Available
functions include ASME steam tables, ASHRAE air properties, orifice flow meter
calculations and select performance functions, including: compressor efficiency
and evaporative cooler effectiveness.
For a complete function listing, available options or a trial version of an
add-in contact T2E3 at 425-821-6036
T2E3 Performance
Analysis Services for Power Plants
Including:
Analysis Tools & Software – from customized spreadsheets to add-ins for Excel or
complete compiled programs, T2E3 can develop software tools and analyses to
support all your performance monitoring needs, including integrating your
existing tools with available site data systems, to create online systems
providing data and results in real-time.
Training – both public seminars and customized options are available. Highly interactive sessions increase
attendees’ knowledge and understanding of the thermodynamic cycles,
instrumentation and analyses needed to improve equipment performance and
reliability.
Performance Test Support – if your site is
required to perform annual capacity or power purchase performance tests, having
Tina Toburen from T2E3 on site to direct the testing can lead to a smoother
test execution with more consistent performance results. Professional reports can also be produced to communicate
the results to all required parties.
Site Marketing and Dispatch Support – Do your marketers and/or
dispatchers understand the operation of your facility? Do they constantly dispatch the plant at
loads which are difficult or impossible to maintain? T2E3 can help you build tools and training
programs to help all parties understand the expected changes in performance due
to ambient conditions and operating constraints. These tools can also lead to a greater
understanding of the long-term economic outlook for your facility.
Unwrap the potential of your
operation. Call for more information on
how we can work together, today!
T2E3
Tina Toburen’s Energy Efficiency Enterprises
14260 120th PL NE, Kirkland, WA 98034
Phone: 425-821-6036
Fax: 253-550-6992
E-mail: performance@t2e3.com
T2E3 Newsletter Volume 2, Issue 4:
December 2008
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