310204e Heat Transfer And Evaporation Flashcards
Define energy
Energy is the ability to do work
Define heat transfer
Heat transfer is the exchange of heat from a hot fluid to a cold fluid to either raise the temperature of the cold fluid or lower the temperature of the hot fluid.
Heat transfer occurs in 3 different ways, what are they?
- Conduction
- Convection
- Radiation
What is thermal conduction?
Thermal conduction is the movement of heat from a region of high temperature to a region of low temperature
What is the equation for expressing the amount of heat that flows through substance per unit of time?
Define convection
Convection is the physical transfer of the stored thermal energy (heat) of a substance from one location to another
Air and water are 2 fluids that store heat and then transfer the heat by convection.
Define radiation
Radiation is heat that transfers through space from a hot object to a cold object in the form of electromagnetic waves
Heat from the sun that reaches the earth warms an object to a temperature higher than the temperature of the ambient air through radiation
True or false
Air requires more than 4 times the mass and 800 times the volume to do the same amount of cooling as water.
True
What is a Condenser?
Condensers are heat exchangers that condense vapor into liquid by removing heat
During condensation latent heat is removed and the temperature and pressure of the condensing fluid should remain the same
What are the 2 types of condensers?
- Surface condensers, where the coolant does not contact the vapours or condensate
- Direct contact condensers, where the coolant, vapour and condensate are in direct contact.
If there is a choice which of the 2 fluids in heat exchanger should be run inside of the tubes and why?
The fluid that has the greatest tendency to foul the tube should be run through the inside of the tube because the inside of the tube is easier to clean.
True or false
Materials that are good electrical insulators are good heat conductors
False, materials that are good electrical insulators are also poor heat conductors
Define Co-current flow
When the fluids in a heat exchanger flow in the same direction
Define counter current flow
When the fluids in a heat exchanger flow in opposite directions
What is the process for tube cleaning in a U-tube shell and tube heat exchanger?
The tubes must be chemically cleaned. If this does not work, the tube bundle must be replaced.
Explain why the floating head in a shell and tube heat exchanger is necessary
The floating hat is necessary to allow the head to move
As the exchanger heats and cools, the tubes will expand and change length
If both ends of the tubes are rigidly fastened, thermal expansion will damage the tubes.
List 4 factors that vary the airflow rate through an air-cooled condenser
- The number of fans running
- The speed of the fans
- The pitch of the fan blades
- The position of the inlet and outlet louvers of the cooler.
What features make the plate heat exchanger a very effective heat transfer device?
The plate heat exchanger uses metal plates to transfer heat.
This makes it effective because the fluids are exposed to a larger surface area
What are 2 common methods for cooling water using an evaporative process?
- Spray ponds
- Cooling towers
Mechanical draft cooling towers can either be an induced or forced draft design. Explain the difference between the 2 designs.
In a forced draft cooling tower, the fan is in the ambient airstream entering the cooling tower and the air is blown through the unit
In the induced draft cooling tower, the fan is located at the exit of the airflow and draws air through the tower.
What does air recycling refer to with cooling towers?
Air recycling is when the wet air expelled at the top of the cooling tower is blown back into the air intake by the wind.
Condensers are classified based on the interaction between the condensing vapor and the cooling fluid
State the 2 classifications
- Surface condensers, where the coolant does not contact the vapours or condensate
- Direct contact, where the coolant, vapour and condensate are in direct contact
What is the objective of evaporation in a process evaporator
The objective of evaporation is to concentrate a solution or slurry feed
Ex. Recovering salt from seawater
What are the 2 main components of a process evaporator
- The heat exchanger
- The vapour body
True or false
Lowering the pressure in the vapor body below atmospheric pressure (vacuum) in the evaporator lowers the boiling point of the water
True
What prevents the liquid from leaving a rising film evaporator along with the vapour?
The deflector plate above the tubes
What evaporator design is used to avoid boiling the product on the heating surfaces?
A forced circulation evaporator.
What serves as the heat source in the second effect in a multiple effect evaporator
The hot vapours produced in the first effect evaporator provide the heat source in the second effect
The water boils in a sequence of vessels where each vessel is held at a lower pressure than the last
Because the boiling temperature of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the next.
Only the first vessel, the one at the highest pressure, requires an external source of heat
What causes lower vapour pressures in evaporators following the first effect
The pressures become progressively lower due to the condensing of the vapours as they move through the heat exchangers
What is the process of crystallization?
Crystallization separates solids and liquids through a process that saturates a solution by lowering its temperature until the solid material precipitates out in the form of crystals.
When are vacuum crystallizers used over standard crystallizers?
Vacuum crystallizers are used when the solution is heat sensitive
What maintains the vacuum in a vacuum crystallizer?
The vacuum is maintained with a steam injector and a barometric condenser.
