Unit Ops 3 Exam 2 - Lecture 6

Flow and Temperature Measurement

Thermocouple

A temperature measurement device consisting of a junction of wires made of two different metals

Thermocouple types and code

Code: ANSI (USA), IEC (elsewhere)

J (black)

K (yellow)

T (blue)

E (pink)

N (orange)

Seebeck coefficient

slope of the measured voltage versus temperature

not a constant

dependent on thermocouple type

Resistance thermometer / Resistance temperature detector (RTD)

Pros over thermocouples:

more accurate

more repeatable

Cons:

slower response time

less durable

more expensive (3x)

narrower temperature range

less common in industry

Platinum resistance thermometer (PRT)

Most common type of RTD

Advantages:

excellent corrosion resistance

widest useful temperature range

stable reading over time

linear temperature vs. resistance

suitable temperature coefficient of resistance (changes enough to read differences)

RTD vs. Thermistor

RTDs use metal

Thermistors use metal oxides

Thermistors may be NTC or PTC

All RTDs are PTC

Negative temperature coefficient (NTC)

May be used as a temperature sensor or as an inrush current limiter

Thermistors

Positive temperature coefficient (PTC)

May be used as a current limiter (resettable fuse). Used in flowmeters

RTDs and thermistors

Temperature measurement compatibility

Thermocouples, RTDs, thermistors, and their wiring are NOT interchangable

RTDs and thermistors require a current source

Thermocouples have a polarity which must be followed and require matching type thermocouple wire all the way to the receiving instrument (wires connected to must match wires inside)

Flowmeters key parameters

required accuracy

phase

temperature and pressure

lubricity

entrained solids/particles

density

viscosity

chemical properties

presence of multi-phases

abrasiveness

rangeability

Rangeability

Also called turndown ratio

highest flowrate measured with acceptable accuracy divided by lowest flowrate measured with acceptable accuracy, expressed as a ratio

If a flowmeter has a useful range from 1 gpm to 10 gpm, the rangeability is 10:1

Positive displacement flowmeters

gear

oval gear

rotary vane

rotary piston

rotary abutment

Non-PD flowmeters

differential pressure flowmeters (orifice, nozzle, venturi, pitot tube)

turbine

electromagnetic

ultrasonic

oscillating

target

thermal mass

coriolis

rotameter

hot wire / hot film anemometer

Gear flowmeter

Liquid enters, is carried around the outside of the gear in the spaces between the teeth, and exits

Measures gear speed

Oval gear flowmeter

Works just about the same as the gear

small flowmeter —> small flowrates

Rotary vane flowmeter

Measures how fast the rotor goes around

Fluid enters, fills space between vanes, transported around the circumference of the housing, exits

Rotary abutment flowmeter

Center rotor rotates in the opposite direction of the other two (displacement rotors)

Low pressure drop

compatible with high viscosity fluids

rotors are kept in sync by a timing gear

gravity driven flows

measure dispensed volume

Displacement rotors do not touch the housing, no friction

identify by triangular shape

Differential pressure flowmeters

Any gain in KE a fluid may acrue due to its increased velocity through a constriction is balanced by a drop in pressure (Bernoulli’s principle)

Orifice, nozzle, venturi, pitot tube

Orifice flowmeter

Highest friction loss

Speeds up through the decreases area, P decreases

Worst rangeability

Nozzle flowmeter

Medium friction loss

Works the same as orifice, but more gradient change in diameter and lower pressure drop

Venturi flowmeter

lowest friction loss

more gradual increase/decrease in diameter

lowest pressure drop

Pitot tube

Differential pressure measured from perpendicular holes

Most common in HVAC and airplanes (portable)

Turbine and paddlewheel flowmeters

use a rotating turbine / paddlewheel with a magnetic sensor to detect the speed of rotation

Paddlewheels may be reversible

Turbines are typically not reversible

The paddle axis is perpendicular to flow

The speed it spins is related to flowrate

Paddle is not sealed therefore not positive displacement

Electromagnetic flowmeters

No moving parts

highest turndown ratio (rangeability)

expensive

fluid must be conductive

uses Faraday’s law of electromagnetic induction

Voltage measured from separation of charges, greater separation, greater voltage, greater velocity (V proportional to v)

Constant magnetic field from coils

Ultrasonic flowmeter

an ultrasonic bean goes first from the downstream transducer to the upstream transducer

The transmission is then sent from the upstream transducer to the downstream transducer

The ultrasound is accelerated slightly by the velocity of the fluid through the pipe

T1-T2 (times) is directly proportional to the liquid flow velocity

Portable, easily retrofitted

pricey

Oscillatory flowmeters

Suitable for both liquids and gases

Fluidic and vortex shedding

Vortex shedding flowmeter

Vortex shedding is caused by fluid flowing around an object (bluff body) placed in the flowing stream

The frequency of the vortices is measured

V_dot = k*d*f(frequency)

The frequency is directly proportional to the volumetric flowrate

Vortices are measured by high frequency pressure transducers, measuring variations in heat transfer from a heated resistor, or ultrasonics

Fluidic flowmeter

Fluid entering flows along one interior wall

Some fluid is diverted back to the inlet (feedback flow) causing the fluid to be pushed against the other wall

Flow shifts from side to side, creating oscillations

Oscillations sensed by an electrically heater thermistor on one side

Alternating flow causes the thermistor to be cooled, this signal is directly proportional to velocity

Target flowmeter

an object (target) is placed in the fluid flow, mounted at a right angle

force exerted on the target is measured by strain gauges, which produce electronic output that is proportional to the square of the flow rate

Optimum size of target depends on the liquid

Thermal mass flowmeter

Only for gases (N2 commonly)

two windings on the sensor tube are heated

when flow takes place, gas caries heat from the upstream to the downstream windings, resulting in a temperature difference, and their resistances change accordingly

The difference in resistance is proportional to flowrate and is measured using a circuit called a wheatstone bridge

Turbulence filter laminarizes flow

gas picks up heat from first winding and picks up less heat from second winding since it’s already heated

Temperature-dependent different in resistance is linearly proportional to mass flow rate

Flowmeter reports volumetric flowrate by converting internally assuming STP (typically wrong)

Coriolis flowmeter

Dual parallel tubes

no flow = in phase

flow = coriolis forces, phase shift, asynchronoous

delta t (phase shift) is proportional to mass flow rate

density is propotional to inverse of frequency squared

calculate V_dot from density and mass flow

Rotameter

Variable area flowmeter

Mechanical

force balance on float

Typically read at the widest point of the float

Narrower at bottom, wider at top

drag force = gravitational force

velocity decreases as area increases, float stops where velocity is equal to terminal velocity

Highest and lowest rangeability

Lowest - differential pressure flowmeters

Highest - electromagentic