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Heating Boiler/Heating System controls
Terms in this set (173)
Most common level control device used in a steam boiler?
A float switch which usually has a low water fuel cutout combined into it.
Purpose of a 3-wire double pole switch? A 2-wire single pole switch?
1) Used as a cut-off switch for the fuel valve and as an alarm.
2) Controls the operation of either the feedwater pump or the feedwater control valve
When the fuel cut off switch opens again, how much water level should read in the gauge glass?
Approximately 20 mm or 3/4"
By code, how many low-water cut outs are required on Heating Boilers?
2 (Low and Low-Low) just like a High Pressure boiler
Heating boilers are typically equipped with a back-up source of water if the primary source fails. Where does this water usually come from?
City water usually. The secondary source is also usually controlled by the low-water cut out device.
What is an Electric feeder Valve?
The electric switch that opens to let city water in if the primary water source fails.
When exactly would the auxiliary city water valve activate?
This supply valve should not open until the water level has dropped well below the cut-in level of the feedwater pump.
2 ways of feeding water to the Steam Heating boiler?
1) Gravity feed with Hartford Loop
Where is the Return line connected to the boiler in a Hartford loop?
At the lowest safe water level in the boiler so the steam pressure will never exceed the pressure from the water level in the boiler.
What would easily define a well balanced steam heating system?
The amount of condensate return is equal to the amount of steam being produced.
Why should boilers with NON-positive displacement pumps use a Hartford Loop?
Because the steam pressure in the boiler could overcome the working pressure of the pump (assuming check valve fails) and a low water condition would result.
Fluctuations of condensate return will affect level of the boiler where the float switch is installed on the boiler. True or False?
False, in cases where the make-up feeder is installed on the condensate receiver. This way a level/supply is always maintained in the receiver and boiler.
If the pump from the receiver to the boiler fails, what would happen?
A low water level situation could occur, so for safe measures a secondary water source float valve could be installed.
2 important requirements of a boiler feedwater system?
- Condensate receiver is of adequate size
- A reliable method of make-up water supply to the receiver
The receiver should be able to hold a volume of condensate equivalent to the water evaporated by the boiler in in a ____ - ____ minute period at normal load.
20-30 minute time period.
Which system should have a bigger condensate receiver, a shopping centre or a high rise building?
A shopping centre due the long horizontal piping used to return the condensate. Large, spread out complexes usually have multiple condensate receivers.
For safe measures, how big should a make-up valve be on a condensate receiver?
Big enough to the supply the boiler at normal load conditions.
Disadvantage of mounting the float valve INTERNALLY on a condensate receiver?
Violent fluctuations in level can occur due to the condensate returning in slugs. Externally mounted float controlled feeder valves with equalizing lines are preferred.
When is it ok to switch the boiler to "high" or "auto" operation?
After the boiler has been properly warmed up.
Small packaged boiler use what kind of control?
A high gas pressure fuel cut off switch Opens or Closes when pressure gets too high?
In what cases is a float-operated feeder valve used as a pilot valve?
In larger systems where a large water supply valve is required.
3 Distinct methods of burner operation control?
- Modulating control
Which process variable is used to start and stop the burner on a boiler?
The boilers Pressure.
Advantages of On/Off control?
- Simplest type of control
- Used on low-capacity boilers
Disadvantages of On/Off control?
- Pressure/Temperature swings between upper and lower set limits
- Inability to operate at maximum efficiency unless continuously fired.
What sets the operating pressure of an on/off control boiler?
The tension of the large spring in the pressure control device. (increase tension = increased operating pressure)
If a single spring is used in a pressure control device, what is the approximate range for cut in/cut out pressure?
5 kPa. This is rather low and can result in cycling, so typically a second spring is used called a differential spring.
What part of the pressure controller actually operates the cut-out point?
The differential adjustment.
What are the two scales used in a pressure controller?
- The setting of the pressure adjustment (cut-in pressure)
- The differential (The difference in pressure between the cut-in and cut-out points)
The differential adjustment and pressure adjustment controls can be used interchangeably. True or False?
Why is a levelling indicator fitted inside the controlled housing?
The pressure controller should be mounted in a perfectly vertical position.
Purpose of the syphon on the pressure controller?
Protect the bellows from steam temperature and corrosive vapours.
ALWAYS INSTALLED THE SYPHON WITH THE LOOP EDGE FACING THE FRONT OF THE CONTROLLER
This will prevent the entire housing from tipping preventing inaccurate mercury measurements.
How many burner nozzles are used in High/low fire control?
2. One nozzle is for low-fire and the other is for high-fire. Each
Under high load demands, how does the high/low fire burner operate?
Low fire will run continuously but when pressure drops low enough the high fire nozzle will kick in until the cut out point (high fire is on/off) .
It is ok to start up the boiler on high-fire auto when its cold. True or False?
False, the boiler should be warmed up on low fire manual then switched to auto.
What happens when the boiler reaches the operating limit setpoint pressure?
The high and low-fire nozzles will close shutting down the boiler.
What must be done to obtain a constant steam pressure output?
The fuel/air must be modulated to change firing rate which will maintain boiler pressure at all demands.
Modulating control requires what 2 pieces of equipment?
- Pressure sensitive modulating controller
- Modulating motor
From varying steam pressure, what purpose do the sliding contacts have from the expanding/contracting bellows?
The sliding contacts vary the resistance and alter the amount of current being sent to the modulating motor which change the air/fuel ratio.
At what pressure does the ASME boiler and pressure vessel code say fuel must cut out at on an automatic fired steam boiler?
15 PSI. (100 kPa)
What safety feature is included on a high pressure cut-out device that lets the power engineer know something is wrong?
A mandatory reset button so the boiler cannot be started until the controller has been reset.
What is the maximum setting the cut-out of the high limit control can be above the safety valve setting?
Approximately 90% above the safety valve setting.
How are bellows type instruments operated on Hot Water boilers?
They do not use steam but instead they use a thermal element which reacts to a volatile liquid under temperature fluctuations.
Maximum Operating temperature on a Hot water Heating boiler?
121 degrees C.
All boilers are equipped with a manually operated start-stop switch. True or False?
What are the three positions of a Damper positioning Switch?
Automatic, High and Low.
Where would you find an Automatic-Manual selector switch?
On boilers equipped with modulating control. Automatic will hold a steady firing rate or Manual selection will let the operator set his own firing rate manually.
When would you use a Flame Control Switch?
When the selector switch is set to Manual on a modulating boiler, the flame control switch increases or decreases the firing rate.
A Low-fire switch is a permissive. True or False?
True, the low-fire switch must be satisfied on start-up to ensure proper mixing according to the air damper.
Purpose of the High Fire Switch?
Prevents the high fire fuel valve from opening when the amount of combustion air is insufficient for high fire rates.
3 safety switches which are used on all boilers with a forced draft fan or blower are....
- Low air pressure fuel cut off
- Low gas pressure fuel cut off
- High gas pressure fuel cut off
If air pressure is too low, is the interlock switch Open or Closed?
Open. When sufficient air pressure is reached the switch closes.
If gas pressure is too high, does it open or close a switch?
A switch opens which shuts valves to the gas lines.
How exactly do you test the High limit cut out controls on a Boiler?
Disconnect the power to the boiler controls and place test leads across the terminals of the operating controls.
Should you attempt to fix a mercury bulb switch?
No, replace it with a new bulb and switch and discard of the failed one properly.
It is ok to attempt maintenance on the Modulating Motor. True or False?
False, it is a closed unit. Replace it with a new unit.
What section of a Hot Water boiler controls the system pressure?
The Head Tank.
Two types of programmers?
Mechanical or Electronic
3 methods of detecting flame failure?
- Flame Rod
- Photoelectric cell
What is a Thermocouple composed of?
2 dissimilar wires welded together at one end to form a measuring or hot junction.
How does a Thermocouple operate?
When the hot junction is exposed to heat its temperature is different than the reference cold junctions. This generates a small voltage (millivolts) which pushes current through a circuit.
What is the primary conductor in a thermocouple?
A copper sheath.
Limit control in a Thermostat circuit?
A safety feature which will shut the fuel off if the temperature gets too high.
Typical voltage generated by a Thermocouple?
No more than 30 millivolts.
Since voltage generated by a Thermocouple is so small, how is sufficient voltage reached to shut off the main gas fuel valves?
A Transformer is used.
What is a Thermopile?
In cases where a bunch of thermocouples are connected in series a sufficient amount of voltage can be generated to shut fuel valves.
How does a Flame Rod/Electrode work?
The flame rod forms a complete electrical circuit through the flame to the burner. When there is no flame, the circuit is open.
Why is a relay required in the electrical circuit of a Flame Rod?
The signal created from the flame is not strong enough to keep the SSV's open so the relay amplifies the signal.
In an AUTOMATIC fired system, what responsibilities does the relay have?
It not only acts as a safeguard device but it also programs the start up and shut down of the firing equipment.
What type of fuel is most common for the use of Flame Rods?
Mostly Gas burners.
How does a Photoelectric cell differ from a Flame Rod?
No electric circuit is conducted. Photoelectric cells react to the presence of Infrared or Ultraviolet rays and "scan" the flame.
What specific type of cell is used in a INFRARED scanner?
A lead-sulphide cell.
Electrical resistance DECREASES with an INCREASE in the amount of infrared light. True or False?
The current flowing through the cell is what generates the actual signal. True or False?
False, it is the voltage that is generated through the cell that is considered to be the "Flame Signal".
The temperature of the flame scanner should not exceed _______ deg.C
50 degrees C
Infrared scanners are used for what type of fuel?
Oil because oil burns yellow.
Operation/Construction of a Ultraviolet scanner?
- Only responds to Ultra-violet rays
- Mounted as close to the flame as possible
- Maximum temperature: 100 deg.C
- The scanner should not be in sight of the igniter
- Also known as a CAD cell
- Consists of a vacuum tube containing a curved metal cathode made of cadmium oxide and an anode wire
- The anode emits electrons when exposed to a flame and completes an electric circuit energizing the SSV's
What type of burners are Rectifying Photocells used?
Gun-type oil burners
Which section of the ASME code describes the procedures for testing flame safeguard devices?
Section VI (6)
Common time for automatic gas valves to close after manual valves are closed?
What should you use to clean contact points on Safeguard devices?
Hard finished paper can be used. Never use a file.
Four main components of a Pneumatic Control System?
- Air compressor with storage tank
- All piping and tubing
- Controlled devices (convector valves, zone valves, damper operators etc.)
Common receiver pressure in a Pneumatic system?
480-550 kPa (70-80 PSI)
To what pressure does the reducing valve drop pressure to?
15 PSI approximately
What is Proportional Action?
When the control pressure varies in proportion to the change in controlled condition, it changes the control pressure.
2 types of Bleed Type controllers?
Normally Open and Normally Closed
When does a Non-bleed type controller activate?
When it reduces the control line pressure.
Typical throttling range of a Modulating Controller?
20-90 kPa or 3-13 PSI.
Where would you see Dual Temperature Thermostats?
In heating systems for day/night control or Heating and Cooling systems for Summer/Winter.
3 Means of control in a Humidistat?
- Human hair element (increase in length)
- Hygroscopic material (same as human hair concept)
- Bi-wood element which consists of 2 types of wood which deform differently pending on amount of moisture absorbed
2 types of Dampers (as final control elements)?
- Opposed blade
- Parallel blade
Where are Parallel blade dampers used?
Wherever there is On/Off or Open and Closed operation.
Where are Opposed Blade dampers used?
They are commonly used for modulation or proportional operation.
2 types of Damper OPERATORS?
Bellows type and Piston Type (piston type is more common)
What type of Auxiliary device would outdoor air intake louvres use?
Electric Pneumatic Relays.
4 pieces of equipment which commonly use Pneumatic-Electric Switches?
How many positions can a Manual Pneumatic selector switch have?
Up to 4 positions.
Pneumatic Gradual Switch?
Basically a very accurate pressure reducing valve equipped with a knob and dial.
3 Way valves allow either ______ or ______ kPa air into a control system.
100 kPa or 125 kPa (15 or 18 PSI)
What is a Cumulator?
Also called a Pneumatic Relay, and similar to a non-bleed type controller but it does not use the direct force of a sensing device. Instead it is activated by the control signal from one or more controllers.
What defines a Self-Contained control system?
The sensor, controller and controlled device form a combined unit. ex: Thermo-Hydraulic Control
In general, A control system uses what to function properly?
A sensing device to change pressures in the control line.
What prevents a piston type damper operator from leaking?
In general, how do Electric Control systems function?
They start and stop a flow of electric current or by varying voltage/current between the controller and the controlled device.
4 Advantages of using electricity for control purposes?
- Readily available
- Wiring is usually simple and easy to install
- signal from sensing elements can produce more than one electro-mechanical output.
- Single controller/actuator combinations are possible
What is an Actuator?
A device used to position dampers and control valves
Setup which allows the continuity of flow of current or stops the flow of electricity.
N.O. (normally open) ?
Contact which completes circuits when a relay is energized
N.C. (normally closed) ?
Contact which completes circuits when a relay is normally de-energized.
30 V or lower.
A device operated by the electrically generated magnetic force to open or close a contact.
What is the most important control in a Heating System?
What kind of switch is installed in a Bimetal Thermostat?
A Two-Position electric switch which has on/off control to regulate the heat supply.
Since the Thermostat should be a compact instrument, how would increase the sensitivity of the Bi-metal strips?
Make them longer and change their shape (U, coils or spirals)
What is the purpose of a Heat Anticipator in a Heating Thermostat?
Gives the Thermostat timed two-position action which prevents the temperature of the room air from overshooting the set temperature of the Thermostat.
When would Open Contact switches be used?
With light duty Thermostats controlling low-voltage circuits.
When would Mercury and Snap switches be used?
For more Heavy duty applications where High voltage/Heavy current control is being used. These quick acting switches prevent burning out of the contact points.
Remote Bulb Thermostat?
- Used to control temperatures in Air ducts, Liquids in tanks or boilers.
- Designed for use with Line Voltage or Low Voltage
- Uses a combination of a sensing bulb/capillary tube/bellows which evaporates a volatile fluid which expands the bellows to an activating lever
Immersion Thermostat? (Aquastat)
- Used to sense the temperature of water or other liquid in various types of tanks/vessels/piping
- The sensing element is liquid filled and activates a switch when the liquid expands/contracts
- Controls temperatures in Ducts or Plenums
- Has a Bi-metal sensing element protected by an open metal guard tube.
- The same as all other Thermostats except on/off control cannot be used.
- A special regulating mechanism is used. Commonly a Variable Potentiometer is used which can vary resistance which control the Voltage within an electric circuit.
- Uses on/off control but is equipped with 2 or more switches which operate in sequence with each switch controlling part of the heat supply.
Where is the best place to install a Thermostat?
- Where it can sense the average Temperature of the area to be heated
- Inside walls, 1.5 m above the floor and in the path of natural circulation as well as the path of forced circulation when the system is not in operation.
What are some places that should be avoided when installing a Thermostat?
- Behind doors, Furniture or Drapes
- By cold drafts from doors or windows
- Next to concealed ducts or piping
- In direct paths of Sun or fireplaces
- Near any other sources of heat
What kind of sensing elements do electronic controlled Humidistats typically use?
Multiple strands of moisture sensitive Nylon
Preventive maintenance for Humidistats?
- Should be kept dust free
- Covers must permit free air circulation over the element
What are H.P. Pressure Controllers measured in?
kPa or mm of Hg.
What are L.P. Pressure Controllers measured in?
mm of H20.
What are the two pressure sources acting on the diaphragm in a Pressure Controller?
One side is the pressure which is to be controlled and the other side is a reference pressure.
Control of Electric Radiator Valves?
- Electrically operated, single-seat valve used for on/off control
- Modulating or 2 position control which controls flow in steam or hot water supply lines
- The motor is a reversible driving motor with reduction gears
- Double seated valve with 3 ports
- Valve is usually actuated by a modulating-motor operator
- Used in Hydronic heating systems for either mixing service or diverting service
Two classifications of Control Motors?
- Motors for Valve operation
- Motors for Damper Operation
One advantage of an electric control system over a pneumatic system?
- The controller can accept a wide variety of inputs
What is an electronic control used for?
Used to position an Actuator.
Open-contact switches, Mercury bulb switches and Snap switches all use bi-metal strips to control electric circuits. True or False?
A heat anticipator uses timed _____ positions action to prevent the room temperature from overshooting.
Electric VS Electronic?
Electric: Starts and stops flow of current to a controller and controlled device.
Electronic: A complete system consists of a sensor, a controller and a final control element.
3 types of control Electronic controllers are capable of?
Advantages of Electronic Control Systems?
- Controllers can be located remotely from the sensors and actuators
- Controllers can accept a wide variety of inputs
- Remote adjustments can be adjusted on central control panels
- Complex controls and Override schemes can be equipped
- Universal type output can be used to interface with many different actuators
Why are Electronic controls well suited to the highly automated processes in HVAC systems?
Because of their reliability and low maintenance and minimal supervision.
What is the exact purpose of a Sensing Element?
Provides the controller with information concerning changing conditions.
Used for continuously changing conditions
Used for fixed state conditions
3 Types of Temperature Sensors?
- RTD's (Resistance Temperature Devices)
Function of RTD's?
- Change resistance with varying temperatures
- Positive temperature coefficient (Resistance increases as temperature increases)
BALCO type RTD?
- Uses a Balco wire using an annealed resistance alloy composed of 70% nickel and 30% iron.
- A 500 ohm BALCO RTD provides a linear resistance variation in a range of -40 to 120 degrees C
Platinum Element Type RTD?
- Linear response and stable over time
- Nominal resistance of 100 Ohms
- A low resistance value could be affected by the element over heating itself
What is a Thermistor?
A solid state discrete sensor with a NEGATIVE temperature coefficient. (As temperature INCREASES, resistance DECREASES)
2 metals used in a Thermocouple?
Iron and Constantan
The milli-volts generated in a Thermocouple are directly proportional to the Temperature. True or False?
Why aren't Electronic Controlled Thermocouples commonly used in HVAC applications?
Because the voltage generated at room temperature is very small.
What are 4 sensing methods that can be used to detect Relative Humidity?
Describe one example of a Resistance type Relative Humidity Sensor.
A sensor uses a layer of Hygroscopic salt such a lithium chloride or carbon powder, deposited between 2 electrodes. The material absorbs and releases moisture as a function of relative humidity, causing a change of resistance of the sensor.
A Pressure Sensor basically acts as a _________ which converts pressure changes into a variable such as voltage, current or resistance.
Voltage range for Transmitter/Transducers?
0-10 Volts DC
Current range for Transmitters/Transducers?
Simple Transformers can be used to adjust current or voltage levels. True or False?
False, more sophisticated circuitry is needed.
After a controller receives a signal and conditions it, what does the output signal do?
The output signal usually goes to an Actuator controlling a final control element.
- Usually Require a specific type or category of input sensors
- They have setpoints and throttling ranges measured in degrees C
Relative Humidity Controllers?
- These controller already receive a converted signal
- Setpoints are measured in % of relative humidity
- Use specific sensors for inputs
- May combine Temperature/Humidity, Dry bulb/Wet bulb temperature etc.
Input circuits can accept one or more of the standard transmitter/transducer signals.
Proportional Integral control as a Control Mode?
A modified modulating control that takes the effect of time into consideration to improve accuracy.
3 types of Output Devices?
- Examples are relays, motor starters and solenoid valves
- They only have 2 discrete states (ex. On/Off)
Modulating Control Devices?
- Use a varying control signal to adjust the final control element
Converts a controller output to a signal that is usable by an Actuator for a final control element.
What would be the purpose of Interfacing an Electronic Control Device to a central System?
For simplicity if any adjustments must be made to setpoints for different controllers.
One distinct advantage of an ELECTRONIC control system?
The controllers can accept a wide variety of inputs
In kPA, an electric to pneumatic transducer would convert a 2 to 10 V D.C. signal to pneumatic signal of ____ - ____ kPa.
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