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| Article: |
Clamp
on measurement using ultrasonic flowmeters for liquids and energy
calculations |
| |
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. |
| 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).
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.
|
Typical system giving the heat flow
in W,kW, MW or BTU
All sensors are clamped external to the piping During measuring |
| |
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. |
Non-invasive Flowmeter with Integrated
Heat Quantity Calculation
| |
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.
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 |
| 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:
|
|
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. |
|
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. |
| |
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.
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. |
| Measurement
of flow from the outside of a pipe with the EESIFLO™
using magnetic clamps |
|
|
| 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.
|
| 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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| Multi-channel
heat flow measurement of 3 thermal energy consumers |
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