Power plants generate electrical power by using fuels like coal, oil or natural gas. A simple power plant consists of a boiler, turbine, condenser and a pump. Fuel, burned in the boiler and superheater, heats the water to generate steam. The steam is then heated to a superheated state in the superheater. This steam is used to rotate the turbine which powers the generator. Electrical energy is generated when the generator windings rotate in a strong magnetic field. After the steam leaves the turbine it is cooled to its liquid state in the condenser. The liquid is pressurized by the pump prior to going back to the boiler A simple power plant is described by a Rankine Cycle.

Rankine cycle

Saturated or superheated steam enters the turbine at state 1, where it expands isentropically to the exit pressure at state 2. The steam is then condensed at constant pressure and temperature to a saturated liquid, state 3. The heat removed from the steam in the condenser is typically transferred to the cooling water. The saturated liquid then flows through the pump which increases the pressure to the boiler pressure (state 4), where ts1.gif (9k) the water is first heated to the saturation temperature, boiled and typically superheated to state 1. Then the whole cycle is repeated.

Typical Modificationsts

Rehaeat

When steam leaves the turbine, it is typically wet. The presense of water causes erosion of the turbine blades. To prevent this, steam is extracted from high pressure turbine (state 2), and then it is reheated in the boiler (state 2') and sent back to the low pressure turbine.

Regeneration

Regeneration helps improve the Rankine cycle efficiency by preheating the feedwater into the boiler. Regeneration can be achieved by open feedwater heaters or closed feedwater heaters. In open feedwater heaters, a fraction of the steam exiting a high pressure turbine is mixed with the feedwater at the same pressure. In closed system, the steam bled from the turbine is not directly mixed with the feedwater, and therefore, the two streams can be at different pressures.

Saturated or superheated steam enters the turbine at state 1, where it expands isentropically to the exit pressure at state 2. The steam is then condensed at constant pressure and temperature to a saturated liquid, state 3. The heat removed from the steam in the condenser is typically transferred to the cooling water. The saturated liquid then flows through the pump which increases the pressure to the boiler pressure (state 4), where ts1.gif (9k) the water is first heated to the saturation temperature, boiled and typically superheated to state 1. Then the whole cycle is repeated.

Typical Modificationsts

Rehaeat

When steam leaves the turbine, it is typically wet. The presense of water causes erosion of the turbine blades. To prevent this, steam is extracted from high pressure turbine (state 2), and then it is reheated in the boiler (state 2') and sent back to the low pressure turbine.

Regeneration

Regeneration helps improve the Rankine cycle efficiency by preheating the feedwater into the boiler. Regeneration can be achieved by open feedwater heaters or closed feedwater heaters. In open feedwater heaters, a fraction of the steam exiting a high pressure turbine is mixed with the feedwater at the same pressure. In closed system, the steam bled from the turbine is not directly mixed with the feedwater, and therefore, the two streams can be at different pressures.

Labels: Thermodynamics, Thermodynamics Cycles

It is often convenient to heat buildings with air. Air heating systems may be cost effective if they can be made simple or if they can be combined with ventilation systems. But be aware that due to the low specific heat capacity of air, the use of air for heating purposes is very limited. Large heat loads requires large volumes of air with huge oversized ducts and fans as results. Transporting huge volumes of air also requires a lot of energy.

Required Air Volume in Air Heating Systems

Required air volume in an air heating system can be calculated as

L = Q / (cp ρ (th - tr)) (1)

where

L = air volume (m3/s)

Q = heat loss from the building (kW)

cp = specific heat capacity air - 1.005 (kJ/kgoC)

ρ = density of air - 1.2 (kg/m3)

th = heating air temperature (oC)

tr = room temperature (oC)

As a rule of thumb the heating supply temperature should be in the range 40-50oC. The air flow should be in the range 1-3 times the room volume.

(1) expressed in imperial units:

L = Q / (1.08 (th - tr)) (2)

where

Q = heat (btu/hr)

L = air volume (cfm)

th = heating air temperature (oF)

tr = room temperature (oF)

Air Heating - Temperature Rise Diagram

The diagrams below can be used to estimate heat required to rise temperature in air flows.

SI units - kW, m3/s and deg C

Imperial units - Btu/h, cfm and deg F

A building with a large room with heat loss 20 kW is heated with air with maximum temperature 50oC. The room temperature is 20oC. The required air volume can be calculated as

L = 20 / (1.005 1.2 (50 - 20))

= 0.55 m3/s

Required air volume in an air heating system can be calculated as

L = Q / (cp ρ (th - tr)) (1)

where

L = air volume (m3/s)

Q = heat loss from the building (kW)

cp = specific heat capacity air - 1.005 (kJ/kgoC)

ρ = density of air - 1.2 (kg/m3)

th = heating air temperature (oC)

tr = room temperature (oC)

As a rule of thumb the heating supply temperature should be in the range 40-50oC. The air flow should be in the range 1-3 times the room volume.

(1) expressed in imperial units:

L = Q / (1.08 (th - tr)) (2)

where

Q = heat (btu/hr)

L = air volume (cfm)

th = heating air temperature (oF)

tr = room temperature (oF)

Air Heating - Temperature Rise Diagram

The diagrams below can be used to estimate heat required to rise temperature in air flows.

SI units - kW, m3/s and deg C

Imperial units - Btu/h, cfm and deg F

- 1 m3/s = 3,600 m3/h = 35.32 ft3/s = 2,118.9 ft3/min (cfm)
- 1 kW (kJ/s) = 859.9 kcal/h = 3,413 Btu/h
- T(oC) = 5/9[T(oF) - 32]

A building with a large room with heat loss 20 kW is heated with air with maximum temperature 50oC. The room temperature is 20oC. The required air volume can be calculated as

L = 20 / (1.005 1.2 (50 - 20))

= 0.55 m3/s

Labels: hvac system

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