|
|
Submitted
Articles: ARTC02012201
Article: Clamp on measurement using ultrasonic
flowmeters for liquids and energy calculations
Submitted
by: EESIFLO
Submit date:22/01/02 |
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
|
|
Multi-channel
heat flow measurement of 3 thermal energy
consumers
|
|
|
|
|
