Steam cycles

Question 1

What is a supercritical cycle, when talking about Rankine processes?

Answer 1

The pressure of the working cycle exceeds critical pressure

Question 2

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.

Answer 2

1. Increase steam temp. and pressure at turbine inlet. (Modern plants: 600C, 250-300 bar)
2. Steam reheat cycles. (Modern plants: single or double)
3. Feed water heating

Question 3

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.

Answer 3

Reduce condenser pressure: depends on the temperature of the cooling medium

Question 4

What else than increased efficiency is the reason for steam re-heating?

Answer 4

Water is formed during turbine expansion, which erodes the blades. Re-heat increases the steam quality, X

Question 5

What is the steel barrier? Answer in temp and pressure

Answer 5

600C, 300 bar. But even higher temperatures and pressures are expected.

Question 6

Name three types of feed water heaters

Answer 6

1. Mixing type (Open type)
2. Surface type (Closed type)
3. Surface type with mixing of drain flow and main flow by pump

Question 7

What is the energy balance of an open feed water heater? (=deareator)

Answer 7

ah_steam,turbine + (1-a)h_condensate = h_out

a = mass fraction of steam from turbine

Question 8

What is the energy balance of a closed feed water heater?

Answer 8

a1*(h1-h2) = h4-h3

a1 = mass fraction of steam from turbine
h1 = steam from turbine
h2 = condensate

h3 = feedwater inlet
h4 = feedwater outlet

Question 9

What does CHP stand for?

Answer 9

Combined heat and power plant

Question 10

What is the equation for EUF? (Energy utilization factor)

Answer 10

(P_el + Q_DH) / Q_in

Question 11

What is the characteristics for a condensing steam turbine plant? (3 things)

Answer 11

• Combined production of heat and electricity
• Condenser pressure as in a conventional plant (around 0.04 bar)
• Wide range of power-to-heat ratio

Question 12

What is the characteristics for a back-pressure steam turbine? (3 things)

Answer 12

• 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

Question 13

What is typical steam data for a biomass plant? (Answer in C, bar). Why is this temperature not higher?

Answer 13

Below 450C and 150 bar. High temperature corrosion accours at 500-700 C: melting point of alkali-rich ashes.

Question 14

What is positive with a biomass plant? (6 possible answers)

Answer 14

• 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

Question 15

What is negative with a biomass plant? (5 possible answers)

Answer 15

• 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

Question 16

Which FWH is thermodynimacal more benefical and why?

Answer 16

An open FWH heater is more thermodynimacal benefical, since the liquid leaves as saturated.

Question 17

How should one as a plant designer balance the number of FWHs? Motivate.

Answer 17

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.

Question 18

What is the ideal thermodynimacal cycle?

Answer 18

The Carnot-cycle.

Question 19

What is the thermodynimacal cycle in a steam cycle called.

Answer 19

The Rankine cycle

Question 20

Name three unwanted constitutes of coal.

Answer 20

Ash, sulfur and moisture.

Question 21

What is HHV an acronym for and when is it used?

Answer 21

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.

Question 22

Describe HV.

Answer 22

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.