Chapter hvac engineering Fundamentals: Part 1 Introduction

230
Chapter Eight
Figure 8.7
Bimetal temperature
sensor.
Figure 8.8
Spiral bimetal tem-
perature sensor.
Figure 8.9
Rod-and-tube temperature sensor.
Design Procedures: Part 6
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Design Procedures: Part 6
231
Figure 8.10
Bulb-and-capillary temperature sensor.
Figure 8.11
Bellows temperature
sensor.
causing a sharp drop in the sensor pressure. This can open a two-
position switch to stop a fan and to prevent coil freeze-up.
The sealed bellows sensor (Fig. 8.11) operates on the same principle
as the bulb-and-capillary sensor. It is usually vapor-filled.
The one-pipe bleed-type sensor (Fig. 8.12) is widely used in pneu-
matic systems. Control air at 15 to 20 lb / in
2
gauge is supplied through
a small metering orifice. A flapper valve at a nozzle is modulated by
one of the previously described temperature sensors or by sensors for
flow, pressure, or humidity. As the valve varies the nozzle airflow, pres-
sure builds up or reduces in the branch line to the controller. By add-
ing appropriate springs and adjustments, this device can also be used
directly as a proportional controller.
Modern electronic control systems use some form of resistance or
capacitance temperature sensor. Widely used is the thermistor, a solid-
state device in which the electrical resistance varies as a function of
temperature. Most thermistors have a base resistance of 3000
⍀
(or
more) at 0
⬚
C and a large change in resistance per degree of temper-
ature change. This makes the thermistor easy to apply, because the
resistance of wire connections (leads) is small compared to that of the
thermistor. Thermistor response is very nonlinear, but circuitry can
Design Procedures: Part 6
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