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Thermal Mass Flow
Meters are based on an operational principle that states that the
rate of heat absorbed by a flow stream is directly proportional to
its mass flow. As molecules of a moving gas come into contact with a
heat source, they absorb heat and thereby cool the source. At
increased flow rates, more molecules come into contact with the heat
source, absorbing even more heat. The amount of heat dissipated from
the heat source in this manner is proportional to the number of
molecules of a particular gas (its mass), the thermal characteristics
of the gas, and its flow characteristics.
Construction/Design
Principle: There are three basic operating methods which are
commonly used to excite the sensor.
| Constant
temperature thermal mass flowmeters,
such as those produced by EPI & Brooks Instrument,
require two active sensors (typically platinum RTDs) that are
operated in a balanced state. One acts as a temperature sensor
reference; the other is the active heated sensor. Heat loss
produced by the flowing fluid tends to unbalance the heated flow
sensor and it is forced back into balance by the electronics.
With this method of operating the constant temperature sensor,
only the skin temperature is affected by the fluid flow heat
loss. This allows the sensor core temperature to be maintained
and produces a very fast response to fluid velocity and
temperature changes. Additionally, because the power is applied
as needed, the system has a wide operating range of flow and
temperature. The heated sensor maintains an index of overheat
above the environmental temperature sensed by the unheated
element. The effects of variations in density are virtually
eliminated by molecular heat transfer and sensor temperature
corrections. These meters typically have a turn down ratio of
1000:1 when properly sized. |
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EPI 8000MP
Series
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Brooks
Instrument
5850 Series MFC
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| Constant
power thermal mass flowmeters are thermal (heat loss)
mass flowmeters and require three active elements. A constant
current heating element is coupled to an RTD. This heated RTD
acts a heat loss flow sensor while a second RTD operates as an
environmental temperature sensor. When the fluid is at rest the
heat loss is at a minimum. Heat loss increases with increasing
fluid velocity. In this method of operation the mass of the
sensor must change it's temperature, making it slow to respond
to fluid velocity changes. In addition, this method of operation
has a limited useful temperature range due to the constant
current applied. The dynamic temperature range may be widened by
applying more power (current) to the heater, but this can result
in excessive heat applied to the heater when the fluid is at
rest. The effects of variations in density are virtually
eliminated by molecular heat transfer and sensor temperature
corrections. These meters typically have a turn down ratio of
100:1. |
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| Calorimetric
or energy balance thermal mass flowmeters require one
heating element and two temperature sensors. Although many
design variations exist, they all have a similar operating
method. Typically the heater is attached to the middle of a flow
tube with a constant heat input. Two matched RTDs or
thermocouples are attached equidistant upstream and downstream
of the heater. The temperature differential at flowing
conditions is sensed, producing an output signal. Because both
temperature sensors see the same temperature and pressure
effects, the design is inherently unaffected by density changes
and the result will be a true mass flow output. Limitations of
this flowmeter design would commonly be a maximum flow rate of
200 liters per minute, non-industrial packaging, and a tendency
to clog in dirty fluids. These meters typically have a turn down
ratio of 10:1. |
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| Coriolis
mass flowmeters provide a direct mass flow measurement
and are easy to size based on process requirements. The
principle of operation is the Coriolis effect or conservation of
angular momentum due to the Coriolis acceleration of a fluid
stream. When an excitation force is applied to a tube causing it
to vibrate, the fluid flowing through the tube will cause a
rotation or twist to the tube because of the Coriolis
acceleration acting in opposite directions on either side of the
applied force. Various tube designs, excitation sources, and
sensors may be incorporated in the Coriolis mass flowmeter.
Coriolis mass flowmeters tend to be much larger is size compared
to other types of flowmeters and, when purchase price is of
concern, Coriolis mass flowmeters may not be an option due to
their high price. These meters typically have a turn down ratio
of 10:1. |
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Brooks
Instrument
Quantim Series Coriolis Meter
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Typical
Applications:
| Automotive:
Compressed air monitoring - Natural gas consumption - Powder
paint air flow - Paint booth/paint oven ventilation |
| Utility
Services: Electric,
gas, water works & sewage plants, for monitoring and control
of: Stack or flue gas - Waste water aeration - Ventilation
systems - Digester gas - Gas flows - Nitrogen purge - Combustion
air - Boiler inlet air |
| Petroleum
& Gas Industries: Custody
transfer - Landfill gas recovery - Flare gas measurement - Gas
mixing - Gas quality studies - Leak testing |
HVAC:
Heating, ventilation
& air conditioning for:
Air balancing - Duct flows - Energy conservation - Fume hoods -
Clean rooms - Laminar flow benches |
| Laboratory/R&D:
Flow research -
Biomedical studies - University studies - Toxicology studies -
Energy studies- Industrial Hygiene - Occupational Safety -
Experimentation |
| Petroleum
& Gas: Custody
transfer - Landfill gas recovery - Flare gas measurement - Gas
mixing - Gas quality studies - Leak testing |
Our Favorite
Application:
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Gas Sub-Metering
Utility Auditing
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In
any industry, finding a better means to monitor and control your
operating expenses is one sure way to improve your competitive
position.
The
ability to monitor the actual amount of gas entering your
facility and to accurately track it's use throughout the
individual areas of consumption, can give you the information
you need to adjust for peak usage and to correctly assign costs
when reviewing general operating expenses and specific product
profitability.
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Georgia, Tennessee & Alabama Only
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Georgia, North
& South Carolina |
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