Temperature control zones

A schematic view of an extruder is shown in figure 1. The extruder barrel is essentially a ferrous alloy cylinder, with aluminum block heaters attached to the outside. There are several temperature control zones along the length of the extruder. Measurement thermocouples are installed in the extruder barrel itself. Barrel temperature is used to control the temperature of the polymer melt. Energy from the heaters is conducted both radially and axially in the barrel. Below, figure 2 shows a sketch of the extruder barrel, with the heaters and the temperature measurement points used in this paper marked. 

A well-referenced thermocouple can be just as satisfactory and will probably become the standard in the future. The reference junction must be in a temperature-controlled zone or ambient compensation must be made. 

Several alternative geometric arrangements, involving different relative dispositions of the components, have been successfully used in thermobalances, in which the essential feature is that the sample is suspended in the temperature-controlled zone. Typical designs of apparatus capable of making mass measurements of heated samples are described in References 4-8, 10, 12, and 16. 

The laboratory fixed-bed microreactor consists of a 60 cm long quartz tube (I.D. 1.0 cm), in which a porous disk supports the catalytic bed. It is surroundwl by an electrical fiimace supplied with three heated and temperature controlled zones. The temperature is monitored by a Chromel-Alumel thermocouple, placed in another quartz tube, coaxial and internal to the reactor, along the whole length of the catalytic bed (1-2 cm). Four mass flow controllers (Brooks) allow to measure the flow rates of high purity gases N2O (0.2% vol.) + He, NO (1% vol.) + He, He (99.995% vol.) and O2 (99.995% vol). 

It is common for an extrusion line to be separated into several temperature control zones. The number of zones depends on the length of the barrel, the type of adapter or transfer line to the die, and the size and complexity of the die. An extruder may have as few as three or well over ten zones. Each zone, or circuit, contains up to four of the following components temperature controller, temperature sensor, heating unit, and cooling unit. Usually only barrel temperature zones utilize a cooling unit. 

An example of grafting maleic anhydride onto polypropylene in a twin-screw extruder was described by Wong (14). Polypropylene was coated with 1.1 wt% maleic anhydride and fed to a 53-mm twin-screw extruder at 30 kg/hr. Four temperature control zones were used, and styrene monomer at 1.4 wt% was injected into the second zone. They obtained 0.8 wt% bound maleic anhydride on the product, and the melt flow index was 17 dg/min. When 500 ppm Lupersol 130 catalyst was added and no styrene was injected, the melt flow index increased to 258 dg/min, indicating signiflcant degradation. No grafting was obtained when an antioxidant was added to the feed. 

The most common methods for heating and cooling the barrels are electrical heating and air or water cooling. The barrel usually is divided into at least three temperature controlled zones, which permit better heating control. 

There are at least eight temperature controlled zones, namely ... 

The whole HR system is divided into temperature control zones temperature measurement and control are carried out with the aid of automatic regulators separated from the mould. 

Last month s trend graph was 24 hrs in the life of a large museum AHU in a northern climate. The constant volume system delivered constant 55°F supply air to terminal reheat coils serving multiple temperature control zones. These temperature control zones were primarily art galleries open to the public during the day. The AHU consisted of a mixing box with airside economizer, heating hot water preheat coil, chilled water coU, humidifier, and supply fan. 

The number of temperature control zones is usually recommended by the mixer manufacturer, with three being quite common, one each for rotors, mixing chamber sides and drop door. Where the door top is profiled to the mixer shape, as is often seen in intermeshing rotor mixers, two zones are commonly adequate, one for the mixing chamber sides and ram, if this is temperature controlled, and one for the rotors and drop door. 

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