Quench Air Control System
QAC

PROBLEM

Monitor and quantify the filament cooling effort (quench) on the cooling effort quench on spun synthetic fibers and effect an accurate control of the quench air flow to regulate a constant quench condition despite variations in supply air temperature, humidity, density, pressure and filter clogging. At the same time monitor related parameters and collect historical data to allow derivation of ideal quench setpoint values.

SOLUTION

Introduce into the quench airstream a sensor which emulates the filament's heat dissipation process and, using closed-loop control of a supply air damper valve, regulate individually the heat dissipation capacity of each diffuser position to a pre-determined setpoint.

THEORY

The objective of the "quench" process is to remove heat energy from the Dacron filament bundle on-the-fly and therefore reduce its temperature from the high level required by the extrusion process to a lower level that will lessen the tendency of the individual filaments to stick together or to any other surface. This is done by forcing air over the surface of the hot filaments. The air absorbs the heat from the filaments and carries it away.

Air is composed of a number of elements and compounds whose molecules vary in mass and thus vary in heat conduction and absorption. As this composite air flow passes over the filaments the air molecules impinge the Dacron molecules and the difference in the temperature (or molecular energy level) between the Dacron and the air causes heat transfer from the warmer to the cooler. The greater the temperature difference the greater the heat transfer. The more air molecules-per-second striking Dacron molecules the greater the heat transfer.

The air flow which is delivered to the filaments is subject to considerable variation which affect its ability to absorb the heat energy. Of course the temperature (or heat energy level) of the air is one factor. The air density defines the space between the molecules. This affects the conduction of the air which, in turn, affects its ability to absorb heat. Air density is a function of its pressure, its temperature, and its composition such as the presence of water vapor (humidity). Besides these variations in the air itself other variations in the delivery rate of the air affect the overall quench process. The mass flow rate (cubic feet per minute) of course affects the "molecules-per-second." The mass flow rate is a function of both its driving force (the supply air pressure) and the resistance it encounters in its path. The diffuser, which actually distributes the air flow to the filaments, is vulnerable to clogging over time. This causes a constantly changing resistance to the air flow.

Its easy to see that without some method of monitoring and control a dependable quench process consistency is impossible. Prior to the development of the Quench Air Control System (QAC) there was no closed-loop control in place. The temperature of the air supply delivered to the machine was held fairly constant and the relative humidity was monitored to watch for intolerable limits. The pressure in the diffuser housing was periodically measured to detect the need for cleaning the accumulated debris from the filter. The result was considerable variations in quench performance from position to position and each was in constant transition as the filter gradually clogged.

The QAC system consists of a number of elements working together under electronic control to provide, for each quench position individually, continuous:

Quench performance monitoring

Duct pressure monitoring

Damper valve position monitoring

Selectable closed-loop process control by any of the above three parameters

Clogged filter alert

Out-of-limits alarm for all parameters

Historical data storage for permanent record or statistical analysis

Failure detection

The elements of the system are:

Quench effect sensor

Pressure sensor

Damper position sensor

Damper valve variable actuator

Control circuit card

Host computer

Alarm control circuit

Alarm annunciator

Keypad display unit

A description of each elements' operation follows. The first five elements listed above are installed and operate autonomously for each of the sixty-four quench positions on the machine. One common host computer and alarm system serves all of the positions collectively.

QUENCH EFFECT SENSOR - The key to regulating the quality of the quench process is in being able to measure what the yarn filament is experiencing. That's what the QES does. The yarn filament gets its heat energy from the extrusion process. As soon as the filament leaves the spinnerette it passes into the diffuser. Here the quench air molecules begin striking the filaments, and since there is no longer a heat energy source the filament's heat is absorbed by the air molecules, and it begins to cool.

The QES is both a heater and a precision sensor. The control circuitry sends to the QES precisely controlled electrical energy which it converts to heat energy. It gets hot, like the yarn filament. The electrical energy is then quickly turned off and with no more energy applied the QES it too begins to cool, since the QES is located in the same airstream as the yarn filaments. The rate of cooling of the QES therefore parallels that of the yarn filaments. The instantaneous electrical resistance of the QES is proportional to its instantaneous temperature so the control circuitry can measure the resistance and plot a curve of the cooling rate. When the QES reaches a certain temperature the electrical energy is turned on again. The amount of energy necessary to return the QES to its original high temperature along with the cooling rate calculations is used to compute the total "quench effect", a true measure of the capacity of the air flow to quench the yarn. All variables (i.e. air temperature, density, humidity, mass flow rate) are taken into consideration by actually measuring the net quench effect they produce.

The "quench effect" is quantified by the control circuit on a scale of 0-500. The value is stored in memory and made available, on a continuously updated basis, for display on either the host computer or the Keypad Display Unit.

PRESSURE SENSOR - A precision electronic, semiconductor-strain-gauge-based air pressure sensor is located in each quench position. It constantly measures the differential pressure between the airstream just before the diffuser and local atmosphere. The maximum measurable value is 5 inches of H2O to a resolution of .01 inches H2O. The control circuit quantifies this measurement on a scale of 0-500. The value is stored in memory and made available, on a continuously updated basis, for display on either the host computer or the Keypad Display Unit.

DAMPER POSITION SENSOR - A variable resistance potentiometer is mechanically linked to the shaft of the damper valve. By voltage division this sensor measures the true position of the damper valve. The control circuit quantifies this measurement on a scale of 0-500. This value is stored in memory and made available, on a continuously updated basis, for display on either the host computer or the Keypad Display Unit.

DAMPER VALVE VARIABLE ACTUATOR - A motor and gearbox assembly is mechanically linked to the damper valve shaft in the quench air supply duct. The motor is driven electrically by the control circuit at variable speed in either direction. If the damper valve shaft is driven fully counter-clockwise the air flow in the duct will be completely stopped. If driven fully clockwise there will be virtually no flow restriction. Thus, the actuator may regulate flow from 0-100%.

CONTROL CIRCUIT CARD - The brains for the Quench Air Control system is a microprocessor-based plug-in circuit board which connects to all the other elements. The inputs from the three sensors allow the card to measure and store the variable conditions. One of the three variables may be assigned to be the "control variable". It will be compared to a programmed setpoint and the actuator driven to increase or decrease the air flow as necessary to maintain the variable at a constant value.

The control circuit also performs a watchdog function to trigger an alarm if any variable exceeds set limits. All of its functions may be monitored by a host computer which can also download a series of variable readings taken over an adjustable time period.

HOST COMPUTER - Any computer capable of RS-422 data communications may be connected to all sixty-four control circuits simultaneously to monitor functions, set parameters and modes, and record variables.

ALARM CONTROL CIRCUIT - Connected to all sixty-four control cards, this circuit detects an out-of-range alarm and triggers a remote common alarm annunciator. This is a secondary or "back-up" to the alarm that the host computer initiates.

ALARM ANNUNCIATOR - This may be any power switched device such as bell, siren, light or any combination. This annunciation may be silenced by the responding party.

KEYPAD DISPLAY UNIT - This portable, battery-powered unit may be connected to an auxiliary socket on the control circuit card while the card is installed in the cabinet. This allows viewing any current variable and viewing or changing any control parameter without the use of the host computer.

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