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Lab #1B Determination of Steam Quality

Lab #1b Determination of Quality Outline • Goal of lab • Phase diagram of • Saturated steam table • Steam quality • Determination of steam quality • Thermos flask • – Fire tube, water tube • Steam trap – Mechanical, thermostatic, thermodynamic 2 Goal of the Lab

• Determination of steam quality (x) in our pilot plant using mass and energy balances • Familiarization with types of

3 Phase Diagram of Water Saturated Within the dome, water exists as steam, Subcooled Liq. + Vap. Saturated vapor which is a mixture of liq liquid and vapor. Liq.2 Const. temp. 5 6 Vap. Here, 3 4 *1 **Saturated steam ***Superheated steam and are Pressure 1: Sub-cooled liquid constant and are Hc Hv 2: SaturatedHv liquid 3: Sat. steam (low qual.) called the saturation 4: Sat. steam (high qual.) temperature and 5: Saturated vapor pressure respectively. Enthalpy (H) 6: Superheated vapor

Left of dome: Subcooled liq; Within dome: Saturated steam; Right of dome: Superheated steam As we move from left to right within the dome, more and more of the water is in vapor phase. The fraction that is in vapor phase is called steam quality (denoted by ‘x’). It varies from 0 to 1 OR 0% to 100%.

Latent heat of vap. (vap) at any temp. or pr. = Hv –Hc at that temp. or pr. 4 Saturated Steam Table

Atmospheric pressure

Temperature (°C)

361.3

What is the latent heat of of water at atmospheric pressure?  = H –H = 2676.1 – 419.04 = 2257.06 kJ/kg vap (at atm. Pr.) v(at atmPr.) c (at atm. Pr.) 5 Steam Quality (x) • The term “Steam” generally refers to saturated steam and not superheated steam • “Steam” is a mixture of liquid (condensate) and vapor

• The enthalpy of steam (Hs) is a weighted mean of enthalpy of condensate (Hc) and enthalpy of vapor (Hv)

•Hs = x Hv + (1 – x) Hc

• Rearranging, x = (Hs –Hc)/(Hv –Hc)

• Note that x = 0 when Hs = Hc

• Also, x = 1 when Hs = Hv

• Higher the steam quality, higher the value of Hs • 0 ≤ x ≤ 1 OR 0% ≤ x ≤ 100%

Note: Hc & Hv at saturation temperature & pressure are determined from steam tables 6 Determination of Steam Quality ms, quality of x; Pressure, P OR Temperature, T Steam m, cp, T: Mass, specific heat, temperature resp. mc, cp(c), Tc mh, cp(h), Th Flask Cold water Hot water Subscripts: ‘c’ for cold; ‘h’ for hot

Introduce cold water of known mass (mc) and temperature (Tc) in a flask Introduce steam into flask till water becomes hot (Th) Determine specific heat of water at Tc and Th from table of

Mass balance: mc + ms = mh Energy balance: mc {cp(c)} Tc + ms (Hs) = mh {cp(h)} Th Determine H from above equation s Use interpolation for: Also, H = (x) H + (1 – x) H s v c cp(c), cp(h), Hc, Hv with Hv and Hc being determined from steam tables at the absolute (not gauge) steam pressure of P or steam temperature of T

Thus, x = (Hs –Hc)/(Hv –Hc) 7 Thermos Flask

Conduction and convection heat loss: Decreased by vacuum, stopper, glass, cork, air Radiation heat loss: Decreased by reflective (silvered) glass 8 Generation of Steam

• Water is converted to steam (in a boiler) using a basic source of energy – Fuel oil, natural , wood, coal, heating element – One of the above is used to generate hot which heat the metallic surface (usually steel) in contact with water to generate steam

9 Fire Tube Boiler

• Hot gases are inside tubes surrounded by water • The heat vaporizes water and converts it to steam • As needed, steam is released from the boiler • Fire tubes boilers are well suited for applications where there are large fluctuations in demand for steam

10 Fire Tube Boiler

11 Water Tube Boiler • Hot gases are outside tubes through which water flows • The heat vaporizes water and converts it to steam • As needed, steam is released from the boiler • Rate of heat transfer is higher in a water tube boiler – This is due to turbulence of water in the tubes • Water tube boilers operate at larger capacities and higher • They are more flexible and safer – Safety relates to phase change taking place in small tubes and not in a large vessel 12 Water Tube Boiler

13 Steam Trap

• What is a steam trap? – A device that separates the liquid and vapor phases of steam • Functions of a steam trap – Prevent steam from escaping through it – Allow condensate to pass through it – Discharge air and any other gas through it

14 Types of Steam Traps

• Mechanical – Based on density difference between vapor and condensate (inverted bucket) • Thermostatic – Based on temperature difference between vapor and condensate • Thermodynamic – Based on pressure difference as vapor condenses

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