| Non-invasive
Flowmeter with Integrated Heat Quantity Calculation
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Thermal energy is mainly distributed
by fluid media to the points of consumption. The
energy manager is not only interested in the total
energy required, but also in the consumption of
individual heat consumers and the flow of energy
in the plant in general. The EESIFLO™ EF
portable ultrasonic flowmeter with integrated
heat quantity calculator has been developed to
compliment permanently installed devices.
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The flowmeter EESIFLO™ EF is especially appropriate
for measurements in large variable supply networks, e.g.
to register the heat distribution in a large complex of
buildings or to review the heat balances in a process
engineering facility. This device is particularly useful
in situations where temporary, non-intrusive inspections
of heat consumption and distribution need to be made quickly.
The advantages of this portable instrument are its flexibility,
enabling it to be used in a wide range of applications,
and the low installation and running costs.
| Principles of
Heat Quantity Measurement |
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The differential
method is the basis for the precise measurement
of heat quantity. This method considers the enthalpy
that enters and leaves a system. The difference
between the two values gives the heat consumption.
Since the enthalpy difference cannot be measured
directly, the value is calculated from the volumetric
flow, the inflow and outflow temperatures and
the heat coefficient for the medium.
Assuming constant conditions, the heat flow can
be calculated with the following formula:
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The heat
coefficient ki is defined by the specific enthalpy
and the density of the heat carrying fluid. These
two quantities depend on the temperature and pressure
of the medium. In incompressible media however,
the variation with pressure is insignificant and
can be ignored. Consequently, flowmeters to measure
the quantity of heat consist of devices for measuring
volumetric flow and temperature. A microprocessor
is necessary to compute the quantity of heat flow.
The newly developed flowmeter EESIFLO™
EF incorporates all these features and in contrast
to conventional flow meters, it allows the user
to measure heat flow and distribution from the
outside of pipes without the necessity of disrupting
the process in the plant. This is achieved by
using a clamp-on ultrasonic flow meter together
with two surface temperature sensors.
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The EESIFLO™
EF ultrasonic flowmeter features two input channels
to connect resistance temperature sensors Pt100
in four wire circuit. This sensor type has been
chosen because of its popularity in industrial
applications and ready availability in a variety
of versions. Two surface sensors are supplied
with the unit to measure the temperature of the
inflow and outflow. The user may, however, connect
other types of sensors of a compatible type according
to specific application requirements. This is
particularly advantageous where temperature sensors
are already installed in the pipe. In such cases,
an input correction for each sensor is required
to obtain a linear resistance temperature curve.
These correction values can be stored in the non-volatile
memory of the flowmeter and are therefore always
available. When using the supplied sensors, the
ability to correct may serve to compensate for
the temperature gradient of the pipe.
The so-called energy temperature, which represents
the temperature for the transportation of energy,
is of special interest for measuring heat flow.
According to Adunka[1], this temperature corresponds
to the temperature in the middle of the pipe
in case of turbulent flow. Under laminar flow
conditions, it is more difficult to determine
this temperature and the energy temperature
is calculated as the mean of the temperatures
of the wall and the centre of the pipe.
When using surface temperature sensors, it
is the pipe wall temperature which is measured
not the energy temperature. In practice however,
the temperature difference is important in the
calculation of heat flow not the absolute temperatures.
The absolute temperatures are only required
to determine the heat coefficients. Studies
at the University of Rostock[2] showed that
the difference between the surface temperatures
approximates to the difference between the energy
temperatures. The pre-condition is, that the
pipe has sufficient insulation to limit the
heat loss through the pipe walls. Both the inflow
and outflow temperatures should always be measured
with the same type of sensor.
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These systems are ideal
for energy efficiency optimization in
industrial sectors and buildings. EESIFLO™
offers a highly accurate, low cost and
robust Energy Management Solution.
The BTU (or Energy) Flow measurement
systems can be readily configured for
almost any size of pipe and are completely
non-intrusive, since all the sensors
are installed on the outside of the
pipes being measured.
Advantages over traditional type flowmeters
are seen by the accuracy ,sensitivity
and longevity of the meters since they
are able to measure both high and low
flow rates with the same accuracy, due
to the fact that the transit time technology
is not dependant on moving parts and
frictional wear and tear.
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Using two additional clamp on temperature
sensors (PT100) or customer temperature
inputs , we are ready to establish the quantity
of heat by a method known as the Differential
Measurement Priniciple.
Our systems can calculate the heat flow
by taking into account the temperature
difference between the inlet and outlet
,the flow at the outlet of the system
in conjunction with some other relevant
properties of the medium (density and
specific heat capacity).
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The co-efficents, which the instrument needs
to know, in order to measure the heat flow of
various media are pre programmed into the flowmeter.In
cases where the temperatures of inflow and/or
outflow are known , or are constant during the
whole measuring period, you may enter these
fixed temperatures manually into the instrument.
In these instances, the temperature sensors
need not be connected.
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the following information is available:
• Volume flow
• Heat flow
• Flow velocity
• Total flow volume or heat quantity (if
total counting activated)
• Temperature T1 (inlet temperature)
• Temperature T2 (outlet temperature)
• Temperature difference T1-T2
EESIFLO™ heat meters give the option of
displaying two of these measured values (one
in each line of the display) and of configuring
the display readings according to your requirements.
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The flow measurement
of the heat carrying fluid is based on the ultrasonic
transit time technique. This method utilises the
transmission of sound waves in the fluid. Sound
pulses are sent alternatively downstream and upstream
through the liquid. The ultrasonic signal has different
transit times for the two directions comparable
to a swimmer in a river who swims faster downstream
than upstream. The resolution of the signal time
difference is 0.1 ns with a transit time of the
sound from 16 µs and 1.6 ms. If these values
together with details of the profile of the pipe
section are known, the volumetric flow rate can
be calculated.
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The transducers for coupling the
sound signals through the pipe clamp from the outside
onto the pipe ensuring that there is no disturbance to
the flow nor any expensive installation costs. This method
of flow measurement implies that the pipe diameter and
tolerances are part of the measuring conditions. Often
the inner diameter and wall thickness of the pipe are
unknown although this information is required to calculate
the volumetric flow from the flow velocity. The input
of incorrect pipe parameters will result in measurement
errors. For this reason, an device for measuring the wall
thickness of the pipe was incorporated into the flowmeter.
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Measurement of flow from the outside of a
pipe with the EESIFLO™ using magnetic
clamps
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| Heat Quantity Calculation |
The microprocessor
within the flowmeter computes the heat flow
from the measured inflow and outflow temperatures
and the volumetric flow rate. The specific enthalpy
and the density of the fluid can be internally
calculated depending on the measured temperature.
As various liquids may be used as heat carriers,
the portable ultrasonic flowmeter EESIFLO™
can be adapted for specific tasks using an
in-built database. The database contains information
on pipe materials and fluids frequently used
and requiring measurement. As well as information
on sound velocity and viscosity, the database
also stores the coefficients necessary for
calculating the heat quantity.
The database can be specifically adapted
and extended by the manufacturer to meet specific
customer requirements. It is also possible
for the customer to enter set-up values and
make changes to the stored data. Special software
has been designed for use with a Personal
Computer to generate the coefficients used
for calculating the heat flow and to transfer
them via a serial interface to the flowmeter
where they are stored in non-volatile memory.
These data are available even when the instrument
has been repeatedly switched off, the batteries
have been changed or a cold start has been
performed.
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The EESIFLO™
EF can measure volume flow, flow velocity,
mass flow or heat quantity of liquids within
a temperature range from -30 °C up to
130 °C. With specially designed high temperature
transducers, the temperature range can be
extended up to 250 °C, and for short periods
up to 300 °C. The ultrasonic sensors are
small, lightweight and very robust. Pipe diameters
may range from 10 up to 3,000 millimetres.
The instrument can always be used where
the pipewall and the liquid to be measured
are sonically conductive. This is true for
pipewalls consisting of homogeneous material,
such as steel, synthetic material, glass
or copper, and for liquids which carry not
an excessive amount of solid particles or
gas bubbles. There is no dependency on electrical
parameters of the fluid such as conductivity
or dielectric constant.
To assist the user in obtaining a complete
profile of the flow conditions in the plant,
the EESIFLO™ EF features an in-built
data logger which can record up to 150,000
measuring values and up to 15 different
sets of site parameters. The data can either
be transferred to a Personal Computer (PC)
or to a printer as numerical values or in
graphic format.
The device allows the operator dialogue
in different languages and guides the user
through the menus for parameter set-up,
measurement or data storage.
The instrument can feature an integrated
measuring point multiplexer which allows
for the connection of up to four independent
flow sensor sets with one transmitter. EESIFLO™
automatically recognises the connected sensors
through Intelligent Sensor Identification.
This means that all calibration parameters
are stored in the sensor and automatically
transferred to the instrument at the time
when the sensors are connected.
EESIFLO™ can also be fitted with various
process inputs and outputs. The instrument
can be equipped with a maximum of four temperature
inputs whereby the temperatures can be freely
assigned to the available flow channels.
This makes it possible to configure, for
example, a 3-channel heat flow measuring
system with a common inlet temperature and
three independent outlet temperatures
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This article has been written
and submitted by Phil Hughes who is the Asian Development
Director for EESIFLO.
For more information contact
EESIFLO’s Asian Headquarters at
EESIFLO 60 Kaki Bukit Place #02-19 Eunos TechPark
Singapore, SG 415979 Singapore
Telephone: +65-6748 6911 Fax: +65-6748 6912 Or visit our
website www.eesiflo.com
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