Fixed temperature detectors

There are two common types of heat detectors - fixed temperature and rate of rise. Both rely on the heat of a fire incident to activate a signal device. Fixed temperature detectors signal when the detection element is heated to a predetermined temperature point. Rate of rise detectors signal when the temperature rises at a rate exceeding a pre-determined amount. Rate of rise devices can be set to operate rapidly, are effective across a wide range of ambient temperatures, usually recycle rapidly and can tolerate a slow increase in ambient temperatures without providing an alarm. Combination fixed temperature detectors and rate of rise will respond directly to a rapid rise in ambient temperatures caused by fire, will tolerate a slow increase in ambient temperatures without effecting an alarm, and recycle automatically on a drop in ambient temperature. 

Thermal detector Senses when temperatures exceed a set threshold (fixed temperature detector) or when the rate of change of temperature increases over a fixed time period (rate-of-rise detector). 

There are two common types of heat detectors. Fixed temperature detectors operate when the detection element is heated to a predetermined set temperature. Rate-of-rise detectors respond when the temperature rises at a rate exceeding a predetermined amount. 

Where process equipment is provided with fixed-temperature detectors, these should be located as near as possible to the potential fire source for example, above flammable liquid pump seals, immediately over a solvent draw-off point, or mounted above a crude tank mixer stuffing box. As a general rule, fixed-temperature detectors directed at a potential hazard should be considered only for process equipment where specific fire problems are anticipated. 

Fixed temperature detectors are preferred because they require less calibration and maintenance. Heat detectors are normally more reliable than other types of detectors because of the simple nature of their operation and ease of maintenance. These factors tend to lead to fewer false alarms. The main disadvantage of heat detectors is that they are unlikely to detect fires in the incipient stage, where little heat is generated, but much smoke is likely.. Since heat detectors are inherently slower in operation than other types of detectors, they should be considered for installation in areas where high speed detection is not required. 

Heat is the most obvious choice of a characteristic by which a fire can be automatically recognized. In the section on fire suppression systems, the fusible links in the sprinkler heads represented one type of heat detector. Alloys have been developed that will have reproducible melting points. When the temperature at the detector site exceeds the melting point of the alloy, contacts are allowed to move so that the device can either make or break a circuit, just as with a manual alarm system. There are plastics which can perform in the same manner. Fixed temperature systems are very stable and not prone to false alarms, but are relatively slow to respond. There are several other versions of these fixed temperature detectors, including bimetalhc strips, where the differential rate of expansion of two different metals causes the strip to flex or bend to either make or break the contact. Others depend upon the thermal erqransion of hquids. 

Heat Detectors There are several kinds of heat detectors. They include fixed temperature detectors, rate-of-rise detectors, rate compensation detectors, and others. 

Figure 25.7 Chubb fixed temperature detector. (Manual of Firemanship. Courtesy Home Office)...

Fixed Temperature Detector A device that responds when its operating element becomes heated to a predetermined level. 

Heat detectors operate by many different approaches, but the most common are fixed temperature detectors that function at a very specific, fixed activation temperature that is set by design (Dungan, 2008). They may be self-resetting or self-destructing, meaning the entire unit must be replaced once it functions. 

Heat detectors can be of either the fixed temperature, rate of temperature rise or linear type. Fixed temperature detectors are activated when the... 

Fixed-Temperature Detectors. Thermostats are one of the most widely used fixed-temperature heat detectors in signaling systems. 

Fixed temperature detector Rate of rise detector Rate compensated... 

One effect that a flaming fire has on the surrounding area is to rapidly increase air temperature in the space above the fire. Fixed temperature heat detectors will not initiate an alarm until the air temperature near the device exceeds the design operating point. The rate-of-rise detector, however, will function when the rate of temperature increase exceeds a predetermined value, typically around 12 to 15°F (7 to 8°C) per minute. Rate-of-rise detectors are designed to compensate for the normal changes in ambient temperature [less than 12°F (6.7°C) per minute] which are expected under nonfire conditions. 

A method was proposed to obtain the kinetic rate constant at a fixed temperature with a one-point measurement. This method is comparable to gas-chromato-graphic concentration measurements and can in principle be executed with a convenient gas-chromatograph equipped with a flame ionization detector (FID) [37]. The background of this method is introduced in the following. 

In the gas chromatographic method (ASTM D 2887), a sample of the test wax is dissolved in xylene and introduced into a gas chromatographic column that is programmed to separate the hydrocarbons in boiling point order by raising the temperature of the column at a reproducible, calibrated rate. When wax samples are used, the thermal conductivity detector is used to measure the amount of eluted fraction. The data obtained in this procedure are reported in terms of percentage recovered at certain fixed temperature intervals. 

The thermocouple is in close proximity to the sample in (b) and is positioned inside, but not in contact with, the sample holder. This arrangement is better than (c) because the thermocouple will respond to small changes of sample temperature. The best method of sample temperature detection is to have the thermocouple either in contact with the sample or with the sample container, as shown in (c). In the latter, the temperature detected will be the integrated temperature. However, the main problem is that with sensitive recording balances the thermocouple leads can cause weighing errors, or at least interference with the balance mechanism. One way to detect the actual sample temperature and yet not interfere with the balance mechanism is to suspend an electronic device near the sample holder which will transmit the sample temperature to a fixed receiver located near the sample container. Manche and Carroll (13) described a unijunction transistor relaxation oscillator which used a thermistor as the temperature detector. The frequency of oscillation, which is a function of sample temperature, was transmitted via a mutual inductance between two suspended coils to a receiver and counter. The device was limited, however, to a maximum temperature of about 150°C. 

Heat detecting devices faU into two categories those that respond when the detection element reaches a predetermined temperature (fixed-temperature types) and those that respond to an increase in temperature at a rate greater than some predetermined value (rate-of-rise types). The two types can be combined into a single instrument They are generally installed when the use of smoke detectors is not practical, or as a backup to smoke detectors. They are used in the following locations ... 

Rate Compensation A rate compensation detector responds to a preset temperature. However, it is less sensitive to thermal lag than a fixed temperature device. The sensor wiU detect the present temperature increasing rapidly and trigger the action when reaching the preset temperature. 

FIGURE 4.83 Spot-type, fixed-temperature snap-disk detector. 

Other Types. Other forms of fixed-temperature heat detectors are the fusible link, occasionally employed to restrain operation of an electrical switch until the point of fusion is reached, and the quartzoid bulb thermostat, which depends on removal of the restriction by breaking the bulb. Both of these units require replacement after operation. 

By using heat detection devices operating on the rate-of-rise or fixed-temperature principle, or other devices such as ultraviolet or infrared detectors designed for individual hazards, it is possible to apply water to a fire more quickly than with systems in which operation depends on opening of sprinklers only as the fire spreads. 

Thermal Methods Level-measuring systems may be based on the difference in thermal characteristics oetween the fluids, such as temperature or thermal conductivity. A fixed-point level sensor based on the difference in thermal conductivity between two fluids consists of an electrically heated thermistor inserted into the vessel. The temperature of the thermistor and consequently its electrical resistance increase as the thermal conductivity of the fluid in which it is immersed decreases. Since the thermal conductivity of liquids is markedly higher than that of vapors, such a device can be used as a point level detector for liquid-vapor interface. 

Catalytic activity for the selective oxidation of H2S was tested by a continuous flow reaction in a fixed-bed quartz tube reactor with 0.5 inch inside diameter. Gaseous H2S, O2, H2, CO, CO2 and N2 were used without further purification. Water vapor (H2O) was introduced by passing N2 through a saturator. Reaction test was conducted at a pressure of 101 kPa and in the temperature range of 150 to 300 °C on a 0.6 gram catalyst sample. Gas flow rates were controlled by a mass flow controller (Brooks, 5850 TR) and the gas compositions were analyzed by an on-line gas chromotograph equipped with a chromosil 310 coliunn and a thermal conductivity detector. 

The catalytic reforming of CH4 by CO2 was carried out in a conventional fixed bed reactor system. Flow rates of reactants were controlled by mass flow controllers [Bronkhorst HI-TEC Co.]. The reactor, with an inner diameter of 0.007 m, was heated in an electric furnace. The reaction temperatoe was controlled by a PID temperature controller and was monitored by a separated thermocouple placed in the catalyst bed. The effluent gases were analyzed by an online GC [Hewlett Packard Co., HP-6890 Series II] equipped with a thermal conductivity detector (TCD) and carbosphere column (0.0032 m O.D. and 2.5 m length, 80/100 meshes), and identified by a GC/MS [Hewlett Packard Co., 5890/5971] equipped with an HP-1 capillary column (0.0002 m O.D. and 50 m length). 

Both multi-residue methods are presented in several parts, which separate general considerations from procedures for extraction, cleanup and determination/ confirmation. Whereas in EN 12393 several extraction and cleanup steps cannot be combined arbitrarily, the modular concept is utilized to a greater extent in EN 1528. In the latter standard, there is no limitation to the combination of several extraction procedures, mostly designed for different commodities, e.g., milk, butter, cheese, meat or fish, with different cleanup steps. Both standards, EN 1528 and EN 12393, do not specify fixed GC conditions for the determination and confirmation. All types of GC instruments and columns, temperature programs and detectors can be used, if suitable. 

Hydrolytic Kinetic Resolution (HKR) of epichlorohydrin. The HKR reaction was performed by the standard procedure as reported by us earlier (17, 22). After the completion of the HKR reaction, all of the reaction products were removed by evacuation (epoxide was removed at room temperature ( 300 K) and diol was removed at a temperature of 323-329 K). The recovered catalyst was then recycled up to three times in the HKR reaction. For flow experiments, a mixture of racemic epichlorohydrin (600 mmol), water (0.7 eq., 7.56 ml) and chlorobenzene (7.2 ml) in isopropyl alcohol (600 mmol) as the co-solvent was pumped across a 12 cm long stainless steel fixed bed reactor containing SBA-15 Co-OAc salen catalyst (B) bed ( 297 mg) via syringe pump at a flow rate of 35 p,l/min. Approximately 10 cm of the reactor inlet was filled with glass beads and a 2 pm stainless steel frit was installed at the outlet of the reactor. Reaction products were analyzed by gas chromatography using ChiralDex GTA capillary column and an FID detector. 

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