Heat Exchanger Flashcards
Describe a gas to gas heat exchanger
Commonly used in compressor operations with intercoolers and aftercoolers. The hot compressed gas runs through the tubes with the cooler atmospheric air simply being blown across the outside fins to supply the necessary cooling
Why does water have the ability to absorb or reject more heat than air?
Because it is more dense than air
Describe a liquid to gas heat exchanger
The hot compressed gas travels around the outside of the tubes while the cooler water or glycol travels through the tubes in the shell and tube heat exchanger
What two methods of removing heat in an air to liquid heat exchanger
Conduction, convection
Describe a liquid to liquid heat exchanger
Both sides of the tubes have a liquid flowing through it in a shell and tube cooler. It is common in many industrial applications
What are some different types of fluid used in cooling processes
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 becomes lighter, moving upward. The heavier, cooler air falls, and takes its place to be heated
How are convection currents increased?
Auxiliary equipment such as pumps or fans. This is called forced convection
What things change the rate of heat transfer in a convection transfer?
The velocity of the fluids inside and outside the tubes is a major factor. The amount of direct contact or scrubbing action also affects
Why is turbulent flow promoted?
It is built into heat exchangers to allow the hot and cold fluids to scrub the walls of the tubing or plates to increase heat transfer rate
What is radiation heat transfer
It requires no medium of transport, and travels across a vacuum. The sun is an example of radiant heat
Which type of heat transfer plays a small part in heat exchangers?
Radiation heat transfer
What are the common flow path arrangements?
Parallel flow, counter flow, cross flow, mixed flow.
Describe a parallel flow path
Both fluids are moving in the same direction, and there is one shell pass, and one tube pass
This is the most inefficient.
Describe counter flow path.
The flow is in the opposite directions. This is the most efficient transfer method
Describe cross flow path
The fluids are introduced perpendicular to each other. Not as efficient as counter flow, but has the benefit of producing more turbulence which makes for greater efficiency.
What two flow paths are commonly combined for greater efficiency?
Counter flow and cross flow
What is a mixed flow path?
Half the time is parallel flow and half the time it’s counter flow
What is the most common heat exchanger?
The finned tube heat exchanger. It is the same style as on a car radiator
What applications would use the finned tube heat exchanger
It would be used on compressor intercoolers and aftercoolers and stationary engine cooling
Where can you find fins on heat exchangers
They are common on the outside of air cooled compressors, but fins can also be added to the tubes inside shell and tube heat exchangers
What is a PHE?
Plate heat exchanger
Describe plate heat exchangers
It is many pressed plates clamped together in a frame. The plates is corrugated with alternating ridges and grooves. The pattern gives the flow a turbulent path, the hot and cold fluids run in opposite directions in the spaces between the plates
How is the spacing on a plate heat exchanger?
Spacing is very important. They are placed between 2-5 mm gaps
Give some advantages of plate heat exchangers
They are low fouling, they have high heat transfer coefficients, they are compact, The reading can be changed easily by burying the number of plates, no insulation required, vibration free
What is another benefit of a plate heat exchanger
They are good for fluids with high turbidity
What is a major disadvantage of the plate heat exchanger
It cannot run with high-pressure
What is a very important factor with the plate heat exchanger
The flow rate is critical
Describe a spiral heat exchanger
It is very efficient due to both fluids having a true counterflow path. The continual curving passages creates high turbulence. It has a long path, so there is much heat transfer opportunity.