180 Questions
What is the main characteristic of direct-fired heaters?
They have burners inside the heater.
In a fractionator reboiler, what is the function of the direct-fired heater?
Heat fluid drawn off the bottom of the fractionator tower.
For what purpose are direct-fired heaters used in preheating the feed to a fractionator tower?
To heat the feed to the fractionator tower.
In what instances are start-up heaters used for preheating the process feed to a reactor?
To get the chemical reaction started.
What role does the fired heater play in chemical reactions?
To promote a direct chemical reaction.
Which arrangement is shown in Figure 1 provided in the text?
Fractionator Reboiler
What is the primary function of a radiant heater?
Transfer heat by radiation to the tubes
What is a common design feature of convection heaters?
Fins on the outer surface of the tubes
In a combination radiant and convection heater, where is the radiant section typically located?
Below the burners and radiant section
What is the main defining characteristic of a vertical, rectangular cabin heater?
Central row of radiant tubes dividing the furnace vertically
What is the primary purpose of fired heaters in heating viscous fluids?
Lower the viscosity of the fluids
What type of fired heater has the lowest efficiency at 40-50%?
Radiant heater
Which section of a combination heater contains the burners and the radiant portion?
Radiant section
What distinguishes a box heater from other fired heater designs?
Box-like shape in the radiant sections
What heat transfer mechanism is primarily used in convection heaters?
Convection
In a radiant heater, how is heat transferred to the tubes after radiation through the furnace space?
By conduction through the tube metal
What is the main function of a direct-fired heater in preheating the process feed to a reactor?
To ensure the process fluid is hot enough for the chemical reaction to occur
What is the purpose of start-up heaters in the context of chemical reactions?
To initiate the chemical reaction by providing the necessary initial heat
In what scenario would a direct-fired heater be considered unnecessary in a chemical plant?
When the chemical reactions are exothermic and self-sustaining
Which of the following best describes the primary role of a fired heater in a fractionator tower?
To heat fluid drawn off the bottom to re-enter the tower in a vapor state
What is a common challenge associated with using direct-fired heaters for chemical reactions?
Inability to reach high enough temperatures for some reactions
Why might a direct-fired heater be shut down after a chemical reaction in some scenarios?
To conserve energy and reduce operational costs
What is the main difference between a radiant heater and a convection heater?
The heating surfaces in a radiant heater are in direct sight of the furnace while in a convection heater, combustion gases flow across the tubes.
What makes combination radiant and convection heaters more common than radiant or convection heaters alone?
Radiant and convection heaters together provide a more balanced and efficient heat transfer process.
How are combustion gases utilized in convection heaters to transfer heat to tubes?
By flowing across the tubes and carrying heat through convection.
What design feature makes the vertical, cylindrical heater the most common fired heater design?
The efficient placement of burners firing upwards into the furnace.
Why do some fluids need to be heated by fired heaters before processing?
To lower their viscosity for easier pumping and processing.
What is the main heat transfer mechanism used in radiant heaters?
Radiation through the furnace space.
What distinguishes a cabin heater from other fired heater designs?
The presence of a sloping transition section in the radiant zone.
Why are box heaters so named in the context of fired heaters?
Because they contain a rectangular shape in the radiant sections.
What is a common feature of convection heaters that aids in heat transfer efficiency?
The presence of fins on the outer surface of the tubes.
Why are convection heaters generally more efficient than radiant heaters?
The presence of fins on the outer surface, improving heat transfer.
What is the primary purpose of the refractory brick and other insulating materials used in the heater?
To keep heat inside the furnace and prevent overheating of the exterior
Where are the vertical gas burners mounted in a typical vertical heater?
In the floor of the furnace
What is the heat transfer mechanism in the convection section of the typical vertical heater?
Convection
What is the main method by which combustion air is received in a typical vertical heater?
By natural draft due to the height of the stack
Where does the heated process fluid go after leaving the lowest tubes on each wall of the vertical heater?
Directly to the process equipment in the plant
In a typical vertical, cylindrical fired heater, what type of section is the radiant section and the convection section respectively?
Radiant section is cylindrical, convection section is rectangular
What is the primary mechanism by which combustion air is received in a typical vertical heater?
Natural draft
Which component of the vertical heater is typically located along the walls of the furnace?
Radiant section tubes
What is the direction of flow of the process fluid through the tubes in the vertical heater?
Descending
What is the purpose of the refractory brick and insulating material in the vertical heater?
To contain heat inside the furnace
What is the typical configuration of the burners in a vertical heater?
Mounted on the floor
What is the typical orientation of the tubes in the radiant section of a vertical, cylindrical fired heater?
Vertical
What type of burners are most common in vertical fired heaters?
Natural draft burners
Where does the secondary air flow in a common gas burner for a vertically fired heater?
Around the perimeter of the burner
What is the purpose of the refractory in a fired heater?
To protect the burner from radiant heat
What is the design feature of a bottom fired vertical heater regarding gas and air mixture?
Not premixed with air
What is the role of adjustable air registers around the burner?
Control excess air limits
How is the gas supplied in a natural gas burner design for a vertical heater?
Piped to burner tips/nozzles
What is the role of the air register in a natural gas burner?
To control combustion air flow to the burner
What happens if a flame scanner fails to see a flame for four seconds?
The burner management system shuts off the fuel to the burner
What is the purpose of the flame scanner in a burner management system?
To ensure fuel to the burner is stopped if no stable flame is present
What does the pressure gauge in a natural gas burner indicate?
The gas supply pressure to the burner
What is the purpose of the pilot in a natural gas burner?
To ignite the main burner
What are the flame scanners responding to in each burner?
The presence or absence of a flame
What is the purpose of the pilot in a natural gas burner?
To ignite the main burner
What is the response of the burner management system if the scanner fails to see a flame for four seconds?
It shuts off the fuel to the burner
What does the pressure gauge in a natural gas burner indicate?
The gas supply pressure
What is the primary function of a flame scanner in a burner management system?
To monitor the flame stability
What is the purpose of the air register in a natural gas burner?
To admit air to the burner
What is the response of the flame scanner if it fails to see a stable flame?
It shuts off the fuel to the burner
What is the purpose of the adjustable air registers located around the perimeter of the burner?
To modify the flame pattern and maintain excess air within limits
What is unique about the design of the natural gas burner shown in Figure 12?
The gas is injected into the combustion zone through nozzles
What is the primary function of the refractory on the furnace floor?
To protect the burner parts from radiant heat
What is the typical configuration of the burners in a vertical heater?
Multiple burners mounted on the walls
What is the purpose of the pilot in a natural gas burner?
To ignite the main burner
What is a common feature of fired heaters that burn fuels that are a product or by-product of the process plant?
They use natural draft burners
What component of the fuel gas supply system protects the section from high pressure in case the upstream regulator fails?
High-pressure switch
In the fuel gas supply system, which equipment reduces the supply pressure and ensures a constant gas pressure to the main fuel control valves?
Pressure regulator
What is the purpose of the main fuel and bypass fuel control valves in the fuel gas supply system?
To control the gas flow at the burner
Why are high-pressure and low-pressure switches included in the fuel gas supply system?
To trip the system offline based on pressure limits
In the fuel gas supply system, which equipment allows monitoring of the pressure upstream of the main fuel control valve?
Pressure gauge
Which component in the fuel gas supply system regulates the fuel pressure to the burners and controls the firing rate of the heater?
Main fuel and bypass fuel control valves
What safety feature in the fuel gas supply system protects the section from high pressure in case the upstream regulator fails?
High-pressure switches
What component regulates the fuel pressure to the burners and controls the firing rate of the heater in the fuel gas supply system?
Pressure regulator
Where does the gas flow after passing through the strainer in the fuel gas supply system?
Into the main fuel control valve
What action do the high-pressure and low-pressure switches take in the fuel gas supply system if pressure limits are exceeded?
Trip the system offline
Which component in the fuel gas supply system allows for manual venting of the gas supply?
Vent valve
What is the purpose of the main fuel and bypass fuel control valves in the fuel gas supply system?
To control the firing rate of the heater
Why should the draft sampling system be airtight?
To ensure accurate pressure readings
What is the purpose of excess air in fired heaters?
To ensure complete fuel combustion
How can the level of excess air in a heater be determined accurately?
Use an online oxygen analyzer
What is the purpose of a flame failure shutdown device in fired heaters?
To prevent fuel supply if no stable flame is detected
Why are fired heaters fitted with an emergency shutdown switch?
To manually activate the shutdown in emergencies
How can the excess air in a fired heater be adjusted to ensure optimal combustion?
Increase the air supply until the flame turns orange
What results from insufficient excess air in a fired heater?
Long yellow or orange flames
How is the correct amount of excess air typically set in fired heaters with forced draft?
Using an oxygen analyzer for flue gas
Why are shutdown devices crucial in fired heaters?
To prevent potential accidents
What do flame scanners in fired heaters primarily detect?
Presence of stable flame
What is the purpose of draft in natural draft heaters?
To create a pressure difference between the inside and outside of the heater
Which locations in the heater generally have thermocouples permanently mounted for monitoring?
In critical areas such as process fluid inlet and outlet, flue gas, and tubes in the hottest section
What is the purpose of draft in natural draft heaters?
To create a lower pressure inside the heater than outside
Where are thermocouples typically mounted in a multiple pass heater?
At each pass along with a common inlet thermocouple
Which section of the furnace might have thermocouples attached to monitor metal temperatures?
Hottest section of the furnace
How is draft measurement typically done in heaters?
With draft gauges, like inclined manometers
In what units is draft usually measured in natural draft heaters?
In inches of mercury or millimeters of mercury
Why are thermocouples permanently mounted in critical locations in heaters?
To monitor metal temperatures and heater performance
Why must the draft sampling system be airtight?
To avoid leaks into the sampling lines which can cause false readings
What is the purpose of excess air in fired heaters?
Ensuring complete combustion of the fuel
Why do most fired heaters lack sophisticated controls for excess air or draft?
To simplify the operation of the heaters
What indicates too much excess air in a fired heater when observing the burner flames?
Shorter than normal flames
Which device in a fired heater system is designed to shut off the fuel supply to burners if a stable flame is not seen?
Flame scanners
What is the primary role of the emergency shutdown switch in a fired heater?
To manually stop the heater in emergencies
Why are pre-alarms installed in fired heaters as part of the safety devices?
To prevent actual trips by alerting the control room
What problem can occur if the process flow and burners are not immediately stopped after a tube failure in a fired heater?
Potential destruction of the heater and danger to surrounding equipment
What is the function of the high stack temperature trip in a fired heater system?
To prevent overheating of the process coils
Which component ensures that the process liquid in fired heaters is not sprayed into the furnace in case of tube overheating?
Shutdown devices
What happens if the flame scanners fail to detect flame at the pilot burners within 10 seconds of the gas valves opening?
The pilot gas will close
What occurs if the main flame is not verified by the flame scanner within a certain time?
The burner gas valve will close
When is the burner management system allowed to switch control of the heater to automatic?
When the main flame is verified by the scanner
What should be done when the burner pressure approaches the low-level trip setting?
The fuel supply system shutoff valve can be closed
What should be done when only one burner remains and the burner pressure is low?
The last burner should be shut off
Why should the fuel supply system shutoff valve be closed after all burners are off?
To prevent fuel leaks
What is the purpose of permissives in a burner management system?
To ensure safe conditions before proceeding with the sequence
How is the completion of the purge in a heater determined in a burner management system?
By testing the stack gases for traces of combustibles
What is the primary reason for prompt pilot ignition in a heater?
To prevent unburned fuel accumulation in the furnace
Why is the control transferred to the automatic control system only when safe to do so in a burner management system?
To ensure a safe operating condition is met
What is the role of pilot ignition in a burner management system?
To prevent unburned fuel accumulation in the furnace
In the context of a burner management system, what are permissives?
Conditions that must be met before proceeding with the sequence
If the pilot flame is not detected by the scanner, what happens next in the burner management system?
The burner gas valve will close and the sequence must be restarted.
What occurs if the burner pressure approaches the low-level trip setting in the heater control process?
One burner is shut off to reduce the firing rate of the heater.
What action will the burner management system take when all burners are shut off and the fuel supply system shutoff valve is closed?
Continuous flow is maintained through the tubes to keep all burners off.
When process conditions permit, what can be done with the last remaining burner in the heater?
Keep it running on minimum firing.
What is the significance of the burner pressure in relation to the functioning of the heater?
Affects the opening of the fuel valves to the main burner.
How is the operation switched from manual to automatic in the burner management system?
Once all main burners are proven and stable.
What must be done before the purge can be started in a burner management system?
The stack damper must be closed
When is the burner management system allowed to transfer control to the automatic control system?
Once the burners are fully lit
What determines the completion of the purge in a burner management system?
Time or testing of stack gases for combustibles
Why must pilot ignition be prompt in a burner management system?
To prevent accumulation of unburned fuel gas
What initiates the purging of a heater in a burner management system?
Permissives being unmet
In the sequence illustrated in Figure 19, what follows the initiation of purging?
Pilot lighting
What is the primary heat transfer process in an indirect fired heater?
Conduction from the heat transfer fluid to the process tubes
Which component of an indirect fired heater helps in creating the required natural draft?
Firetube
What is the purpose of the heat transfer media used in an indirect fired heater?
To transfer heat from the burner to the process tubes
Where is the burner typically connected in an indirect fired heater?
To the inlet of the firetube
Which fluid is NOT a common heat transfer media used in indirect fired heaters?
Methanol
What is the purpose of the expansion tank in an indirect fired heater?
To accommodate the volume changes in the heat transfer media
What is the purpose of glycol in water bath heaters in colder climates?
To reduce the freezing point of the media
Where is the process coil located in a steam bath heater?
In the steam space above the water level
What type of heat transfer oil is commonly used in oil bath heaters?
Special heat transfer oil
What is the advantage of using salt baths over oil baths in heaters?
Salt baths have a higher melting point than oil
What component in the salt bath mixtures helps protect coils from corrosion and pitting?
Potassium nitrate
What process fluid outlet temperature range is achievable with oil bath heaters?
Up to 260°C
What is the primary advantage of using salt baths over oil baths in heaters?
Higher process outlet temperatures
What is the purpose of glycol in water bath heaters in colder climates?
To reduce the freezing point of the media
What is the maximum process outlet temperature that can be achieved using steam bath heaters?
110°C
What is the typical composition of the salt mixture used in salt bath heaters?
Sodium nitrite, potassium nitrite, and potassium nitrate
What is the primary advantage of using oil bath heaters over water bath heaters?
Uniform heat transfer to process fluids
What is the maximum steam pressure that can be safely handled by steam bath heaters?
100 kPa
In indirect fired heaters, what is the immediate source of heat for the process fluid in the heating tubes?
Heat from the burner firetube
What role do the smooth bends of the firetube play in indirect fired heaters?
Promote a low pressure drop
In indirect fired heaters, where is the operating liquid level usually maintained?
Above the process tube bundle
What is the purpose of the expansion tank in indirect fired heaters?
Accommodate temperature changes
Which media are commonly used for different temperature applications in indirect fired heaters?
Glycol and salt solutions
How do indirect fired heaters transfer heat to the process fluid heating tubes?
Via immersion in an intermediate fluid
What indicates insufficient air in a burner flame?
A yellow flame
How is the temperature of the fluid passing through the coil regulated?
By adjusting the thermostat setting
What happens when the main burner reaches the desired heat transfer fluid temperature?
It cycles on and off to maintain the temperature
How is the heater shut down?
By turning off the fuel supply valve
What should be observed during normal heater operation?
The burner operation, process outlet temperature, and heater condition
What happens to the residual gas in the burner manifold during shutdown?
It is burnt off
What is the typical temperature set point for a glycol heater?
80 to 90°C
What is the purpose of preheating the fuel in an indirect fired heater?
To improve combustion efficiency
What is the role of the thermostat in an indirect fired heater?
To sense the heater bath temperature
What is the primary fuel used in indirect fired heaters?
Natural gas or propane
What is the correct sequence of steps to start up an indirect fired heater?
Check components, inspect fuel pressure regulator, and open the main inlet gas valve
What is the purpose of checking the condition of all heater components before start-up?
To identify potential safety hazards
How should the burner flame be adjusted for optimal operation?
By decreasing the gas flow and increasing the secondary air intake
What should be done if a yellow flame is observed on the burner?
Adjust the air damper
How is the fluid temperature regulated within the heating coil?
By adjusting the thermostat setting
What indicates that the main burner has reached the desired heat transfer fluid temperature?
The flame becomes a stable blue flame
How can the heater be safely shut down?
Turn off the fuel supply valve and then block the process stream
What should be monitored when the process outlet temperature is checked often?
The level of the heat transfer fluid
What is the purpose of the pilot in an indirect fired heater?
To provide continuous heat and keep the pilot lighted at all times
What is the recommended operating temperature range for a glycol heater?
80°C to 90°C
What should be ensured before starting the heater in terms of components?
All fittings, connections, piping, and tubing are tight and leak-free
What regulates the gas pressure at the main burner for an indirect fired heater?
The main burner fuel pressure regulator
What is the main criterion for switching the main burner on/off in an indirect fired heater?
Heater bath temperature sensed by the thermostat
Which component is responsible for igniting the pilot in an indirect fired heater?
Portable igniter
Explore the various applications of direct-fired heaters in refineries and chemical plants, specifically in heating process fluids and fractionator reboiling. Learn how direct-fired heaters are utilized to efficiently heat fluids for different industrial processes.
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