Objective 03: Principles and Use of Calorimeters Flashcards
What is problematic about steam being excessively wet?
- Causes corrosion and erosion in equipment
- Reduces plant thermal efficiencies
- Increases risk of water hammer in piping
- May damage equipment that uses the steam
How is the problem of excessively wet steam properly assessed?
Monitor the dryness fraction of the steam at various locations in the system
Calorimeter
Device that is used to measure the dryness fraction of wet steam and to determine the amount of heat in the steam
What are the 3 types of Calorimeters?
- Throttling
- Separating
- Electric
What general steam quality are Throttling Calorimeters used for?
Low moisture content
What principle are Throttling Calorimeters based on?
Enthalpy remains constant when steam pressure is reduced through an orifice or constriction because there is no opportunity for heat to be removed from the steam
What happens to the moisture in steam when it is throttled through Throttling Calorimeters?
The moisture flashes into superheated steam at the suddenly lower pressure
Where should a steam sample be taken in the boiler to analyze steam quality?
As close as possible to
1) the steam user, e.g. steam engine, heater, turbine, etc.; or
(2) the steam source (boiler outlet
How does a simple Throttling Calorimeter work?
- Steam is throttled though the orifice into a lower pressure chamber
- The temperature is measured with a thermometer and the pressure is measured with a manometer
- The steam is allowed to escape to a vent
How is the effectiveness of Throttling Calorimeters ensured?
The body of the calorimeter and the piping to it are well insulated to prevent heat loss to the surroundings and an inadvertent increase in the moisture of the sample
What must be done before a Throttling Calorimeter can start measuring dryness fraction?
It must be warmed with steam for considerable amount of time before the test to ensure the body is at the same temperature as the steam. If the orifice is sized correctly, the steam will be at atmospheric pressure after throttling
What dryness fraction are Throttling Calorimeters used for?
From 0.965 dryness at 700 kPa to 0.93 dryness at 2700 kPa.
If the dryness fraction is below 0.93, how does this affect the usage of Throttling Calorimeters?
A Separating Calorimeter is often required upstream of the Throttling Calorimeter to remove most of the moisture before the steam enters the Throttling Calorimeter
Why is having excessively wet steam problematic for Throttling Calorimeters?
If the steam is too wet entering the Throttling Calorimeter, it may still be wet after throttling rather than dry and containing superheat. It will also be at the same saturation temperature as the entering steam and, without measurable superheat, it won’t be possible to determine the enthalpy.
How does a simple Separating Calorimeter work?
- The steam enters and changes direction as the steam passes through a tube with holes.
- Inertial forces causes the moisture to collect in the cavity.
- The amount of liquid collected is measured in the sight glass.
- The dryer steam exits.
