Sensor Installation Handbook Preface 3rd edition - Siemens ...

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Practical tips 84 Basic circuit: If the air temperature downstream of the heat exchanger falls below the preset temperature, then: ■ the supply/extract fans switch off ■ the outside air/exhaust air dampers close ■ the heating coil valve opens ■ the internal pump switches on ■ the frost alarm trips. The frost alarm is self-locking, to prevent the system from being switched on again automatically by the frost thermostat. The alarm must be acknowledged locally (remote acknowledgement is prohibited). To prevent overheating inside the unit, the heating coil valve is regulated by the frost protection thermostat while the frost alarm is active. Tips for avoiding nuisance frost alarms: The main cause of a frost alarm is a drop in temperature. It is possible, how - ever, for a frost thermostat to trip even when the heat supply is adequate. This may be due to one of the following circumstances: a) Changes in load, such as a change from one fan-stage to another, or a heat recovery system coming on/off load b) Temperature stratification c) System start-up after a shutdown To prevent the frost thermostat from tripping in situations where the temperature is adequate, the following basic circuits have proven effective: ■ Preventive frost protection thermostat ■ Stabilised start function ■ Setpoint-regulated start-up control
Preventive frost protection thermostat: The preventive frost protection thermostat operates in two phases. Frost alarm Open Closed Valve Temperature Example: 1. If the air temperature around the frost thermostat drops below 6 °C, the thermostat takes over control of the heat exchanger valve to maintain the air temperature at a value between 2 °C and 6 °C. 2. If the air temperature is still below the 2 °C switch-off point, (e.g. due to insufficient heating), the frost alarm will trip. Practical tips 85
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Sensor Installation Handbook Prefac
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Sensor Installation Handbook Prefac
Page 9 and 10:
General notes for all sensors Alway
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Always allow a sufficient length of
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Temperature Immersion sensors · Wa
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Water-resistant seal Chilled water
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If the active length (a) of the sen
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Sensors mounted without immersion p
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Temperature Probe sensors · Air En
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The ambient temperature at the devi
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Temperature Averaging sensors Allow
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Do not bend the capillary too tight
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When installing on "drawer"-type un
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Use spacing clips to maintain a 50
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Do not install on external walls. A
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Temperature Outdoor sensors The sys
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Seal gaps between cable/plastic tub
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Temperature Surface-mounted strap-o
Page 41 and 42: Temperature Wind sensors Install on
Page 43 and 44: Avoid shade (from trees or neighbou
Page 45 and 46: A test hole must be provided for ev
Page 47 and 48: Distance for adiabatic humidificati
Page 49 and 50: Humidity sensor Average humidity me
Page 51 and 52: Seal gaps between cable/plastic tub
Page 53 and 54: Where there is a risk of condensati
Page 55 and 56: Avoid using tips which protrude int
Page 57 and 58: Where more than one sensor is used,
Page 59 and 60: Pressure Liquids Pressure tapping p
Page 61 and 62: Pressure measurement in conjunction
Page 63 and 64: Flow monitoring Water Correct insta
Page 65 and 66: Avoid recesses (e.g. shelving) and
Page 67 and 68: Floats: These are used in open and
Page 69 and 70: Principles of operation General All
Page 71 and 72: Example of response characteristic
Page 73 and 74: Stepless devices Stepless devices (
Page 75 and 76: Bimetal rod and tube Material Const
Page 77 and 78: Electrical resistance sensors Metal
Page 79 and 80: Thermocouples These consist of two
Page 81 and 82: Absolute humidity x g H 2O per kg o
Page 83 and 84: Electronic systems Piezoelectric el
Page 85 and 86: Measuring water velocity and flow L
Page 87 and 88: Room temperature measurement Room s
Page 89 and 90: Calibration of measuring sensors Th
Page 91: Practical tips Frost protection the
Page 95 and 96: Pressure control in VAV systems The
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