Steam cycles Flashcards
What is a supercritical cycle, when talking about Rankine processes?
The pressure of the working cycle exceeds critical pressure
One way to increase efficiency in a steam cycle is to increase the temperature of the heat addition. Name 3 ways how that can be done.
- Increase steam temp. and pressure at turbine inlet. (Modern plants: 600C, 250-300 bar)
- Steam reheat cycles. (Modern plants: single or double)
- Feed water heating
One way to increase efficiency in a steam cycle is to decrease the temperature of the heat rejection. Name 1 way how that can be done.
Reduce condenser pressure: depends on the temperature of the cooling medium
What else than increased efficiency is the reason for steam re-heating?
Water is formed during turbine expansion, which erodes the blades. Re-heat increases the steam quality, X
What is the steel barrier? Answer in temp and pressure
600C, 300 bar. But even higher temperatures and pressures are expected.
Name three types of feed water heaters
- Mixing type (Open type)
- Surface type (Closed type)
- Surface type with mixing of drain flow and main flow by pump
What is the energy balance of an open feed water heater? (=deareator)
ah_steam,turbine + (1-a)h_condensate = h_out
a = mass fraction of steam from turbine
What is the energy balance of a closed feed water heater?
a1*(h1-h2) = h4-h3
a1 = mass fraction of steam from turbine h1 = steam from turbine h2 = condensate
h3 = feedwater inlet h4 = feedwater outlet
What does CHP stand for?
Combined heat and power plant
What is the equation for EUF? (Energy utilization factor)
(P_el + Q_DH) / Q_in
What is the characteristics for a condensing steam turbine plant? (3 things)
- Combined production of heat and electricity
- Condenser pressure as in a conventional plant (around 0.04 bar)
- Wide range of power-to-heat ratio
What is the characteristics for a back-pressure steam turbine? (3 things)
- Production of steam (or useful heat) for use in process industry
- High exit pressure in low pressure turbine (more than 1 bar)
- Employed if electricity demand is low
What is typical steam data for a biomass plant? (Answer in C, bar). Why is this temperature not higher?
Below 450C and 150 bar. High temperature corrosion accours at 500-700 C: melting point of alkali-rich ashes.
What is positive with a biomass plant? (6 possible answers)
- Proven technology
- Broad power range available
- Seperation between fuel and thermal cycle possible: getting rid of ashes and contaminants
- Co-firing with fossil fuels possible to enable high efficiency
- Multipurpose plants can increase fuel efficiency
- CO2 neutral generation
What is negative with a biomass plant? (5 possible answers)
- Low thermal efficiency
- High specific investments for small plants
- Large variations in fuel quality
- Expensive fuels
- Cogeneration is required for good economy - DH system needed
Which FWH is thermodynimacal more benefical and why?
An open FWH heater is more thermodynimacal benefical, since the liquid leaves as saturated.
How should one as a plant designer balance the number of FWHs? Motivate.
There should be one OFWH and the rest CFWHs, even though OFWHs are thermodynimacal more benefical. If “n” OFWHs are employed, “n+1” pumps are required. Usally 9 FWHs are employed in modern steam cycles and the pump work would then become significant.
What is the ideal thermodynimacal cycle?
The Carnot-cycle.
What is the thermodynimacal cycle in a steam cycle called.
The Rankine cycle
Name three unwanted constitutes of coal.
Ash, sulfur and moisture.
What is HHV an acronym for and when is it used?
Higher Heating Value. It is used if the temperature of the flue gases are lower than the temperature of devolatilization. Water will then condesate in the combustion chamber.
Describe HV.
The heating value (HV) of a fuel is the amount of heat released by a mole of fuel when it is completely burned and the products of combustion are cooled to the original fuel temperature.
