Fire Detectors - Heat Detectors

1.0 FIRE DETECTION

Delay in detection of fire leads to issues like

Difficulty in controlling fire in the incipient stage with first aid fire fighting appliances
Delayed evacuation process
Delayed the initiation of fire fighting Which may lead to
Increased loss of life and property.

Automatic fire detection is important in unattended/unoccupied premises during working hours, or off hours/holidays.

A fire can be detected by sensing the presence of one of the product of combustion such as

Heat
Radiation
Smoke, etc.

1.1 CLASSIFICATION OF DETECTORS

Fire detectors can be classified in different ways.

Based on the shape/ Range of Detection, they can be classified as:

Spot/point/block type detector: The detector respond only to the conditions existing at the place they are located. [IS 2175 - Block Detector; NFPA - Spot detector]
Line type detector: The detector may be several meter long and will respond along its length.
Volume detector/ surveillance Detector: The detector will respond to a flame anywhere within a volume.

A practical detector must be robust, reliable and sensitive. It should also have long life, should not give false alarms and should be easily tested and readily replaced.

Based on the Principle of Operation, detectors can be classified as:

Smoke or Combustion Product Detectior.
Flame Detector
Heat Detector (Conduction/Convection)
Fire gas detector
Other fire detectors

1.2 HEAT DETECTORS

Heat detectors work under the principle of sensing the heat energy produced by the fire. When the heat energy is sensed at a pre-determined or present condition, the detector will be activated.

Heat detectors are simple in design, reliable and cheap compared to other detectors. However, the sensitivity of heat detectors is poor.

The fire has to be quite large before it can be detected by heat detector.

Depending on the method of detection/operation heat detectors are classified into:

Fixed temperature detector
Rate of rise detector
Themistor probe type detectors

1.2.1 Fixed Temperature Detector

Fixed temperature detectors are designed to activate alarm when the temperature of the operating element reaches a specified point.

As heat transfer from the air to the operating element requires time (mostly by convective method), the surrounding air temperature will always be higher than the operating temperature of the device. This difference between the operating temperature of the device and the actual air temperature is known as thermal lag. The thermal lag is proportional to the rate at which temperature is rising.

Thermal lag describes a material's thermal mass with respect to time. A material with high thermal mass (high heat capacity and low conductivity) will have a large thermal lag.

Expansion of materials with temperature has been considered as a reliable mechanism to devise detector elements of this type. There are different methods by which a fixed temperature sensing elements can be made, namely by:

using Bimetallic strip
using fusible alloy
using heat sensitive cables
using expansible liquid

1.2.1.1 Fixed temperature detector using bimetallic strip

When two metals having different coefficients of thermal expansion are bonded together and then heated, differential expansion causes banding or flexing towards the metal having the lower expansion rate. This action breaks or makes a circuit, causing the detector to activate.

Low expansion metal- invar (alloy of nickel (36%) and iron (64%)

High expansion metal – alloy of

Manganese –copper-nickel; Nickel-chromium-iron; Stainless steel

Bi-metals are used in the form of Strips and Snap disc

All bi-metal elements are self-restoring type and hence detections made of this principle need not require replacement or adjustment

1.2.1.2 Fixed temperature detector using fusible alloy

A fusible alloy is used as solder to keep an element (spring, metal strip etc) in the detector under tension. When the temperature of the solder reaches its melting point, it melts and the element will close/break the contact and initiates an alarm.

This is a spot type detector and its main drawback is that it can be operated only once and is not reusable.

The commonly used alloy is Eutectic metal (-alloys of bismuth, lead, tin and cadmium) which has an approximate melting point of 68 °C.

1.2.1.3 Fixed temperature detector using heat sensitive cables

They are line type detectors. In this case, two current carrying wires are kept separated by a heat-sensitive insulation. The insulation gets softened at its rated temperature and thus making the wires to have electrical contact.

1.2.1.4 Fixed temperature detector using expansible liquid

A quartzoid bulb (bulb in the shape of quartz) is filled with highly expansible liquid with a small amount of gas inside. As the temperature increases, the liquid expands and the pressure of liquid causes the bulb to burst, leading to the activation of alarm.
This type of detections are used in water sprinkler system and are available at different temperature ranges (49 °C, 60, 74, 121, 160, 204, 283 °C)

1.2.2 Rate of rise detector

The rate of rise detector will respond to the rate at which the temperature rises. At a slow temperature rise it will not respond quickly unless the temperature reaches a pre- set value. Different principles are used to achieve this response.

1. Pneumatic rate of rise tubing type heat detector

This is a line type detector
The tube A (small diameter copper tube) is fastened in a continuous loop to ceilings or walls and terminates at both ends in chamber B having flexible diaphragms C, which control the electrical contact D.
When air in the tubing expands under the influence of heat, pressure builds within the chambers, causing the diaphragms to move and close a circuit to alarm transmitter E.
Small and calibrated vents F compensate for small pressure changes in the tubing brought about by small changes in temperature in the protected spaces.

2. Pneumatic Rate of Rise cum Fixed Temperature- Spot Type

In spot type, there is an air chamber and the expansion of air in this chamber due to temperature rise moves a diaphragm and this in turn closes a circuit. The alarm will be activated once the circuit is activated. A calibrated compensating vent within the chamber allows the normal fluctuations in the temperature.

A thermostat device fitted to the detector system allows the activation of alarm in case the temperature rise is at slow rate. The thermostat can be set to a predetermined temperature (57.2 °C- 82.2 °C)

The air in the chamber A expands more rapidly than it can escape from vent B.
This causes pressure to close electrical contact D between diaphragm c and contact screw E.
Fixed-temperature operation occurs when fusible alloy F melts, releasing spring G, which depresses the diaphragm closing contact points.

1.2.3 Thermistor Probe type (spot type- rate of rise type)

Thermistor is a semiconductor with a negative temperature coefficient- that is, its electrical resistance decreases with increase in temperature.

The change in resistance of these thermistor is very large. For example, at 20 °C, its resistance is 100 kΩ while at 100 °C it has only 100 Ω resistance.

The circuit within the detector is designed to alarm when the change in the resistance reaches a pre-set value.

A fixed temperature setting can also be provided in the system so that the system can detect slow temperature rise, if any.

Advantages of thermistor type over fusible link type heat detector

Thermistor Type	Fusible link type
Large coverage area (40m²)	Less coverage area (10m²)
Will not malfunction due to vibration, as they are in solid state	During melting, vibrations may cause problems
Reusable	Cannot be reused
Long life	Fusible links may deteriorate
More sensitive and fast response	Less sensitive and sluggish
Desired temperature can be adjusted	Fixed temperature type

1.2.4 Rate Compensated Heat Detector

The rate of rise device are triggered by rate of increase in ambient temperature and are subjected to false alarm caused by harmless thermal gradient such as flow of warm air from processes (oven, etc). To overcome this difficulty, rate compensated detectors are used.

Here, the outer shell of the detector is made of rapidly expanding alloy stainless steel, which is sensitive to the changes in the surrounding air temperature.
The inner struts are made of alloy having lesser co-efficient of expansion.
At slow rate of heat development, both outer shell and inner strut expands almost evenly and contact is established at pre-set value.
Under a rapid rate of rise in temperature, the shell expands at a faster rate and alarm is activated earlier when the air temperature reaches its pre-set value.
A transient temperature variation may heat up the outer shell, but may not cause alarm.

1.2.5 Performance of Heat Detector

Ref. IS: 2175 :1988)- specification for heat sensitive fire detector for use in automatic fire alarm system (reaffirmed Nov. 1995).

The code specifies that:

Heat detectors should operate at a temperature of 70 ± 7 ° C when constantly put in temperature for 50 seconds in air stream at the rate of 80 cm/s.
In the case of fixed temperature type detector, the detector shall not operate when the air flow is maintained just below 62 °C and shall operate within 50 seconds when suddenly placed in test temperature maintained at 77 °C.
When tested under standard conditions (at 27 ± 2 ° C and air stream velocity of 80 cm/s) the detector must respond as per the details specified in the following table:

Sl. No.	When tested at 27 ± 2 ° C and with an air stream velocity of 80 cm/s,
Sl. No.	the detector must operate within the time (Minutes)	when tested at a temperature rate of rise of (°C/ minute)
1	35 to 50	1
2	11 to 18	3
3	6 to 11	5
4	3 to 6	10
5	1.5 to 3.25	20
6	1 to 2.5	30