Heat Exchanger Flashcards
What are two methods of removing heat in an gas to liquid heat exchanger
Conduction, convection
Describe a liquid to liquid heat exchanger
Both sides of the tubes have a liquid flowing it in a shell and tube cooler. It is common in many industrial applications
What are some different types of fluid used in cooling process?
Gas to gas
Liquid to gas
Liquid to liquid
What are the three basic means of heat transfer?
Conduction
Convection
Radiation
What is conduction?
The transfer of heat from one source to another through direct contact
What things affect the rate of heat transfer by conduction?
Difference of temperature
Area of contact
Conductivity of the metal wall
Nature of the substance
Define convection.
It is a result of the movement of a gas or liquid, from a warm location to a cooler one
Describe an air convection cycle
Air is heated it expands and become lighter, moving upward, then heavier, cooler air falls, and takes its place to be heated
How are convection currents increased?
By auxiliary equipment such as pumps or fans. This is called forced convection
What changes the rate of heat transfer in a convection transfer?
The velocity of the fluids inside and outside the tubes
Direct contact or scrubbing action
Why is turbulent flow important?
It allows the hot and cold fluids to scrub the walls of the tubing or plates,to increase heat transfer rate
What is radiation?
Electromagnetic waves moving across space and requires no medium of transport and can travel across a vacuum
What are the common flow path arrangements?
Parallel flow
Counter flow
Cross flow
Mixed flow
Describe a parallel flow
Both fluids are moving in the same direction and there is one shell pass and one tube pass
(this is the least efficient)
Describe counter flow
The flow is in the opposite directions.
this is the most efficient
Describe cross flow
The flows are perpendicular to each other.
NOT as efficient as counter flow, but has the benefit of producing more turbulence which makes for greater efficiency
Describe mixed flow
Mixed flow is common in heat exchangers with baffles that cause the flows to switch between parallel and counter flow as they progress through out the exchanger
What two flow paths are commonly combined for greater efficiency?
Counter flow
Cross flow
What is the most common heat exchanger?
Finned tube heat exchanger
For what applications is the finned tube heat exchanger use?
Intercoolers
Aftercoolers
Stationary engine cooling
Where can you find fins on heat exchangers?
On air cooled compressors fins are common, but they can be added to the tubes inside the shell and tubes heat exchangers to increase efficiency
What is PHE?
Plate heat exchanger
Describe the Plate heat exchanger
Is a number of pressed metal plates clamped together on a frame with gaskets between the plates.
Hot and cold fluids run in opposites directions.
What is the spacing on a plate heat exchanger?
2-5 mm gap
Give some advantages of plate heat exchangers
Low fouling High heat transfers coefficients Compact No insulation required Vibration free
What is a major disadvantage of the plate heat exchanger?
Limited to temperatures and pressures, high pressure drops
Describe a spiral heat exchanger
It has a long path permitting plenty of exchange heat while keeping radiation losses to minimum. Spiral heat exchanger is very efficient due to both fluids having a true counter flow path
How much surface area of cooling can be provided by a 1 meter diameter X 1.5 meter long spiral heat exchanger?
100 meters
What are cooling towers commonly used for?
To recycle cooling water for large scale applications like air conditioning and power generation
What is the principle of a cooling tower?
The water to be cooled is pumped to the top of the tower, where it is sprayed through nozzles to give a fine spray.
What does the efficiency of the cooling tower depend on?
Area of water surface (small droplets give larger surface area)
Length of contact line
Relative humidity
Cooling towers are divided into two classes what are they?
Natural draft
Mechanical draft
Describe the natural draft cooling tower
Air movement through the tower is dependent on wind conditions. They operate effectively in large open areas with a steady wind.