STEAM

Steam as medium to Generate Power ---Why?

• Steam is used in the following Industries:– Power generation– Sugar– Paper– Fertilizer– Refineries– Petrochemicals– Chemicals– Food– Synthetic fibres – Textiles

Characteristics of Steam

• Highest Specific Heat and latent Heat• Highest heat transfer coefficient• Easy to control and distribute• Cheap and Inert• Non Toxic, odorless

Steam

• The Heat required to change temperature a substance is called as “Sensible Heat” mentioned as “hf” with unit of Kcal/Kg

• If 1 kg of water at 25 degC ie: containing the heat value of 25 Kcals is heated by adding another 75 Kcals, the water is brought to it’s boiling point of 100degC.

• To Change the water to steam an additional 540 Kcals would be required and this heat required to change the water from liquid state to gaseous state is Called as “Latent Heat” without any change in temperature.

• When the steam condenses back to water, it gives up the enthalpy of evaporation which it had acquired for change in phase from water to steam. The enthalpy of evaporation is measured in Kcal/Kg and the symbol ishfg

• The temperature at which water boils is known as Boiling Point or Saturation Temperature.

Heat of steam Calculation

• The total heat of the fully saturated steam is = hf+hg• If the steam has some wetness then dryness will be

mentioned as ‘F’ , now the total heat will be as =hf+Fxhfg• Let us say boiler is at 8 kg/cm2 and steam saturation

temperature is 170 degC, then the total heat of dry saturated steam is

• hf+hg =171.35+489.46 =660.81 Kcal/Kg( values from steam tables)

• If dryness fraction is 96% ( wetness is 4%) then =171.35+0.96x489.46 = 641.23 Kcal/Kg.

Hf (A-B) Liquid Enthalpy: hfg (B-C) Latent heat of evaporation addition at Constant temperature and (C-D) as superheat addition after it attains dry

saturated steam condition.

Two phase region- Wet steam

Liquid region

Critical Point – meeting point of saturated Liquid and Saturated vapor lines.

Superheat region

Saturated Liquid line

Saturated vapor line

A

BC

D

Enthalpy

Temperature

Critical Point

• The Point at which the saturated liquid line and saturated vapour line meets..known as CRITICAL POINT.

• As the pressure rises the latent heat of evaporation also decreases and at point it is zero , from water changes directly to saturated steam .

• The critical point occurs @374.15 DegC & 221.2 bar(a) for steam. Above this pressure the steam is termed as Supercritical and no well defined boiling point applies.

• Above the critical point, only Gas exist, the gaseous state is the most diffusive state in which the molecules have an almost unrestricted motion, and the volume increases without limit as the pressure is decreased.

Steam Distribution • As condensate has very small volume than steam ,

condensation generates pressure drop.• Steam traps are to be part of steam distribution system, for

example a 100mm well lagged pipe of 30 meters length carrying steam at 7 kg/cm pressure can condense nearly 10 kg of water in one hour if the condensate not removed through the traps/ drains.

• Pipe shall have fall gradient of 12.5 mm in 3 m length in the direction of flow.

• Steam traps, steam separators, drain pots @every 30 to 50 m of pipe length) , automatic vents, thermal expansion loops etc are necessary for perfect steam distribution system.

• Proper sizing of steam pipelines will help to reduce the pressure drop.

• The velocities of steam – Wet or Exhaust steam : 20 to 30 m/sec– Saturated Steam : 30 to 40 m/sec– Superheated steam : 50 to 70 m/sec

• Pressure drop in steam pipes Hf = 4fLu2/2gD where Hf = head loss due to friction in m, f is friction factor

dimensionless, L is length in m, u is flow velocity in m/sec, g is gravitational constant (9.81 m/s2) and D is the pipe diameter in m.

Steam Traps (ST)• Purpose of ST is to obtain fast heating of the product or

equipment by keeping steam lines free of condensate, air and non condensable gases. It is a device like valve discharges only condensate and air from the line without discharging the steam.

• Functions are To discharge condensate as soon as formed. Not to allow steam to escape. To be capable of discharging air and other non condensable gases.

• Steam traps of three basic types as per ISO-6704:1982. Thermostatic ( Operated by changes in fluid temperature) Mechanical ( Operated by changes in fluid density) Thermodynamic ( Operated by changes in fluid dynamics)