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jav
jav • Jan 21, 2009

Ways to increase the pressure of steam

LET YOUR HEART BE IN PEACE

Hi, Sir, i wanna method for increasing the pressure of the steam, can you guide me in what all ways can i do that, I will be glad if you guide me through this.. thank you and eagerly awaiting for your reply.....
by
javeed
raj87verma88
raj87verma88 • Jan 21, 2009
Steam is a fluid. A diffuser can be used to increase pressure of a fluid.
raj87verma88
raj87verma88 • Jan 21, 2009
Steady flow equation gives

h1 + (V1)^2/2 +Z1g + dQ/dm = h2 + (V2)^2/2 + Z2g + dWx/dm

dQ/dm=0 , dWx/dm=0 , change in potential energy = 0

New equation
h1 + (V1)^2/2 = h2 + (V2)^2/2

V2 or velocity at outlet is very small as compared to V1 the velocity at inlet and can be ignored.
Magesh.mx
Magesh.mx • Jan 21, 2009
The Saturated Steam Table with properties as boiling point, specific volume, density, specific enthalpy, specific heat and latent heat of vaporization

Absolute
pressure Boiling point Specific volume (steam) Density (steam) Specific enthalpy of liquid water (sensible heat) Specific enthalpy of steam
(total heat) Latent heat of vaporization Specific heat
(bar) (oC) (m3/kg) (kg/m3) (kJ/kg) (kcal/kg) (kJ/kg) (kcal/kg) (kJ/kg) (kcal/kg) (kJ/kg)
0.02 17.51 67.006 0.015 73.45 17.54 2533.64 605.15 2460.19 587.61 1.8644
0.03 24.10 45.667 0.022 101.00 24.12 2545.64 608.02 2444.65 583.89 1.8694
0.04 28.98 34.802 0.029 121.41 29.00 2554.51 610.13 2433.10 581.14 1.8736
0.05 32.90 28.194 0.035 137.77 32.91 2561.59 611.83 2423.82 578.92 1.8774
0.06 36.18 23.741 0.042 151.50 36.19 2567.51 613.24 2416.01 577.05 1.8808
0.07 39.02 20.531 0.049 163.38 39.02 2572.62 614.46 2409.24 575.44 1.8840
0.08 41.53 18.105 0.055 173.87 41.53 2577.11 615.53 2403.25 574.01 1.8871
0.09 43.79 16.204 0.062 183.28 43.78 2581.14 616.49 2397.85 572.72 1.8899
0.1 45.83 14.675 0.068 191.84 45.82 2584.78 617.36 2392.94 571.54 1.8927
0.2 60.09 7.650 0.131 251.46 60.06 2609.86 623.35 2358.40 563.30 1.9156
0.3 69.13 5.229 0.191 289.31 69.10 2625.43 627.07 2336.13 557.97 1.9343
0.4 75.89 3.993 0.250 317.65 75.87 2636.88 629.81 2319.23 553.94 1.9506
0.5 81.35 3.240 0.309 340.57 81.34 2645.99 631.98 2305.42 550.64 1.9654
0.6 85.95 2.732 0.366 359.93 85.97 2653.57 633.79 2293.64 547.83 1.9790
0.7 89.96 2.365 0.423 376.77 89.99 2660.07 635.35 2283.30 545.36 1.9919
0.8 93.51 2.087 0.479 391.73 93.56 2665.77 636.71 2274.05 543.15 2.0040
0.9 96.71 1.869 0.535 405.21 96.78 2670.85 637.92 2265.65 541.14 2.0156
1 99.63 1.694 0.590 417.51 99.72 2675.43 639.02 2257.92 539.30 2.0267
1.1 102.32 1.549 0.645 428.84 102.43 2679.61 640.01 2250.76 537.59 2.0373
1.2 104.81 1.428 0.700 439.36 104.94 2683.44 640.93 2244.08 535.99 2.0476
1.3 107.13 1.325 0.755 449.19 107.29 2686.98 641.77 2237.79 534.49 2.0576
1.4 109.32 1.236 0.809 458.42 109.49 2690.28 642.56 2231.86 533.07 2.0673
1.5 111.37 1.159 0.863 467.13 111.57 2693.36 643.30 2226.23 531.73 2.0768
1.5 111.37 1.159 0.863 467.13 111.57 2693.36 643.30 2226.23 531.73 2.0768
1.6 113.32 1.091 0.916 475.38 113.54 2696.25 643.99 2220.87 530.45 2.0860
1.7 115.17 1.031 0.970 483.22 115.42 2698.97 644.64 2215.75 529.22 2.0950
1.8 116.93 0.977 1.023 490.70 117.20 2701.54 645.25 2210.84 528.05 2.1037
1.9 118.62 0.929 1.076 497.85 118.91 2703.98 645.83 2206.13 526.92 2.1124
2 120.23 0.885 1.129 504.71 120.55 2706.29 646.39 2201.59 525.84 2.1208
2.2 123.27 0.810 1.235 517.63 123.63 2710.60 647.42 2192.98 523.78 2.1372
2.4 126.09 0.746 1.340 529.64 126.50 2714.55 648.36 2184.91 521.86 2.1531
2.6 128.73 0.693 1.444 540.88 129.19 2718.17 649.22 2177.30 520.04 2.1685
2.8 131.20 0.646 1.548 551.45 131.71 2721.54 650.03 2170.08 518.32 2.1835
3 133.54 0.606 1.651 561.44 134.10 2724.66 650.77 2163.22 516.68 2.1981
3.5 138.87 0.524 1.908 584.28 139.55 2731.63 652.44 2147.35 512.89 2.2331
4 143.63 0.462 2.163 604.68 144.43 2737.63 653.87 2132.95 509.45 2.2664
4.5 147.92 0.414 2.417 623.17 148.84 2742.88 655.13 2119.71 506.29 2.2983
5 151.85 0.375 2.669 640.12 152.89 2747.54 656.24 2107.42 503.35 2.3289
5.5 155.47 0.342 2.920 655.81 156.64 2751.70 657.23 2095.90 500.60 2.3585
6 158.84 0.315 3.170 670.43 160.13 2755.46 658.13 2085.03 498.00 2.3873
6.5 161.99 0.292 3.419 684.14 163.40 2758.87 658.94 2074.73 495.54 2.4152
7 164.96 0.273 3.667 697.07 166.49 2761.98 659.69 2064.92 493.20 2.4424
7.5 167.76 0.255 3.915 709.30 169.41 2764.84 660.37 2055.53 490.96 2.4690
8 170.42 0.240 4.162 720.94 172.19 2767.46 661.00 2046.53 488.80 2.4951
8.5 172.94 0.227 4.409 732.03 174.84 2769.89 661.58 2037.86 486.73 2.5206
9 175.36 0.215 4.655 742.64 177.38 2772.13 662.11 2029.49 484.74 2.5456
9.5 177.67 0.204 4.901 752.82 179.81 2774.22 662.61 2021.40 482.80 2.5702
10 179.88 0.194 5.147 762.60 182.14 2776.16 663.07 2013.56 480.93 2.5944
11 184.06 0.177 5.638 781.11 186.57 2779.66 663.91 1998.55 477.35 2.6418
12 187.96 0.163 6.127 798.42 190.70 2782.73 664.64 1984.31 473.94 2.6878
13 191.60 0.151 6.617 814.68 194.58 2785.42 665.29 1970.73 470.70 2.7327
14 195.04 0.141 7.106 830.05 198.26 2787.79 665.85 1957.73 467.60 2.7767
15 198.28 0.132 7.596 844.64 201.74 2789.88 666.35 1945.24 464.61 2.8197
16 201.37 0.124 8.085 858.54 205.06 2791.73 666.79 1933.19 461.74 2.8620
17 204.30 0.117 8.575 871.82 208.23 2793.37 667.18 1921.55 458.95 2.9036
18 207.11 0.110 9.065 884.55 211.27 2794.81 667.53 1910.27 456.26 2.9445
19 209.79 0.105 9.556 896.78 214.19 2796.09 667.83 1899.31 453.64 2.9849
20 212.37 0.100 10.047 908.56 217.01 2797.21 668.10 1888.65 451.10 3.0248
21 214.85 0.095 10.539 919.93 219.72 2798.18 668.33 1878.25 448.61 3.0643
22 217.24 0.091 11.032 930.92 222.35 2799.03 668.54 1868.11 446.19 3.1034
23 219.55 0.087 11.525 941.57 224.89 2799.77 668.71 1858.20 443.82 3.1421
24 221.78 0.083 12.020 951.90 227.36 2800.39 668.86 1848.49 441.50 3.1805
25 223.94 0.080 12.515 961.93 229.75 2800.91 668.99 1838.98 439.23 3.2187
26 226.03 0.077 13.012 971.69 232.08 2801.35 669.09 1829.66 437.01 3.2567
27 228.06 0.074 13.509 981.19 234.35 2801.69 669.17 1820.50 434.82 3.2944
28 230.04 0.071 14.008 990.46 236.57 2801.96 669.24 1811.50 432.67 3.3320
29 231.96 0.069 14.508 999.50 238.73 2802.15 669.28 1802.65 430.56 3.3695
30 233.84 0.067 15.009 1008.33 240.84 2802.27 669.31 1793.94 428.48 3.4069

Example - Boiling Water at 100oC, 0 bar Atmospheric Pressure
At atmospheric pressure (0 bar g, absolute 1 bar ), water boils at 100oC, and 417.51 kJ of energy are required to heat 1 kg of water from 0oC to its evaporating temperature of 100oC.

Therefore the specific enthalpy of water at 0 bar g (absolute 1 bar ) and 100oC is 417.51 kJ/kg, as shown in the table.

Another 2 257.92 kJ of energy are required to evaporate 1 kg of water at 100oC into 1 kg of steam at 100oC. Therefore at 0 bar g (absolute 1 bar) the specific enthalpy of evaporation is 2 257.19 kJ/kg, as shown in the table.

Total specific enthalpy for steam:

hs = 417.51 + 2 257.92

= 2 675.43 kJ/kg

Example - Boiling Water at 170oC, 7 bar Atmospheric Pressure
Steam at atmospheric pressure is of a limited practical use because it cannot be conveyed under its own pressure along a steam pipe to the point of use.

At 7 bar g (absolute 8 bar), the saturation temperature of water is 170.42oC. More heat energy is required to raise its temperature to saturation point at 7 bar g than would be needed if the water were at atmospheric pressure. The table gives a value of 720.94 kJ to raise 1 kg of water from 0oC to its saturation temperature of 170oC.

The heat energy (enthalpy of evaporation) needed by the water at 7 bar g to change it into steam is actually less than the heat energy required at atmospheric pressure. This is because the specific enthalpy of evaporation decreases as the steam pressure increases. The evaporation heat is 2046.53 kJ/kg according the table.

Note! Because the specific volume also decreases with increasing pressure, the amount of heat energy transferred in the same volume actually increases with steam pressure.

If we conside steam to follow ideal gas laws then :

PV=nRT
In the present case we want to keep V constant ,so to incease pressure either Temperature is to be increased or n means number of moles is to be increased. That is density of steam needs to be incresed.
SteamAs the temperature increases and the water approaches its boiling condition, some molecules attain enough kinetic energy to reach velocities that allow them to momentarily escape from the liquid into the space above the surface, before falling back into the liquid.

Further heating causes greater excitation and the number of molecules with enough energy to leave the liquid increases. As the water is heated to its boiling point, bubbles of steam form within it and rise to break through the surface.

Considering the molecular structure of liquids and vapours, it is logical that the density of steam is much less than that of water, because the steam molecules are further apart from one another. The space immediately above the water surface thus becomes filled with less dense steam molecules.

When the number of molecules leaving the liquid surface is more than those re-entering, the water freely evaporates. At this point it has reached boiling point or its saturation temperature, as it is saturated with heat energy.

If the pressure remains constant, adding more heat does not cause the temperature to rise any further but causes the water to form saturated steam. The temperature of the boiling water and saturated steam within the same system is the same, but the heat energy per unit mass is much greater in the steam.

At atmospheric pressure the saturation temperature is 100°C. However, if the pressure is increased, this will allow the addition of more heat and an increase in temperature without a change of phase.

Therefore, increasing the pressure effectively increases both the enthalpy of water, and the saturation temperature. The relationship between the saturation temperature and the pressure is known as the steam saturation curve (see Figure 2.2.1).

Fig. 2.2.1 Steam saturation curve
Water and steam can coexist at any pressure on this curve, both being at the saturation temperature. Steam at a condition above the saturation curve is known as superheated steam:
Temperature above saturation temperature is called the degree of superheat of the steam.
Water at a condition below the curve is called sub-saturated water.
If the steam is able to flow from the boiler at the same rate that it is produced, the addition of further heat simply increases the rate of production. If the steam is restrained from leaving the boiler, and the heat input rate is maintained, the energy flowing into the boiler will be greater than the energy flowing out. This excess energy raises the pressure, in turn allowing the saturation temperature to rise, as the temperature of saturated steam correlates to its pressure.


Enthalpy of evaporation or latent heat (hfg)

This is the amount of heat required to change the state of water at its boiling temperature, into steam. It involves no change in the temperature of the steam/water mixture, and all the energy is used to change the state from liquid (water) to vapour (saturated steam).

The old term latent heat is based on the fact that although heat was added, there was no change in temperature. However, the accepted term is now enthalpy of evaporation.

Like the phase change from ice to water, the process of evaporation is also reversible. The same amount of heat that produced the steam is released back to its surroundings during condensation, when steam meets any surface at a lower temperature.

This may be considered as the useful portion of heat in the steam for heating purposes, as it is that portion of the total heat in the steam that is extracted when the steam condenses back to water.


Enthalpy of saturated steam, or total heat of saturated steam

This is the total energy in saturated steam, and is simply the sum of the enthalpy of water and the enthalpy of evaporation.

Equation 2.2.1
Where:
hg = Total enthalpy of saturated steam (Total heat) (kJ/kg
hf = Liquid enthalpy (Sensible heat) (kJ/kg)
hfg = Enthalpy of evaporation (Latent heat) (kJ/kg)

The enthalpy (and other properties) of saturated steam can easily be referenced using the tabulated results of previous experiments, known as steam tables.
Top The saturated steam tablesThe steam tables list the properties of steam at varying pressures. They are the results of actual tests carried out on steam. Table 2.2.1 shows the properties of dry saturated steam at atmospheric pressure - 0 bar g.

Table 2.2.1 Properties of saturated steam at atmospheric pressure
Example 2.2.1
At atmospheric pressure (0 bar g), water boils at 100°C, and 419 kJ of energy are required to heat 1 kg of water from 0°C to its saturation temperature of 100°C. Therefore the specific enthalpy of water at 0 bar g and 100°C is 419 kJ/kg, as shown in the steam tables (see Table 2.2.2).

Another 2 257 kJ of energy are required to evaporate 1 kg of water at 100°C into 1 kg of steam at 100°C. Therefore at 0 bar g the specific enthalpy of evaporation is 2 257 kJ/kg, as shown in the steam tables (see Table 2.2.2).
However, steam at atmospheric pressure is of a limited practical use. This is because it cannot be conveyed under its own pressure along a steam pipe to the point of use.

Note: Because of the pressure/volume relationship of steam, (volume is reduced as pressure is increased) it is usually generated in the boiler at a pressure of at least 7 bar g. The generation of steam at higher pressures enables the steam distribution pipes to be kept to a reasonable size.

As the steam pressure increases, the density of the steam will also increase. As the specific volume is inversely related to the density, the specific volume will decrease with increasing pressure.

Figure 2.2.2 shows the relationship of specific volume to pressure. This highlights that the greatest change in specific volume occurs at lower pressures, whereas at the higher end of the pressure scale there is much less change in specific volume.

Fig. 2.2.2 Steam pressure/specific volume relationship
The extract from the steam tables shown in Table 2.2.2 shows specific volume, and other data related to saturated steam.

At 7 bar g, the saturation temperature of water is 170°C. More heat energy is required to raise its temperature to saturation point at 7 bar g than would be needed if the water were at atmospheric pressure. The table gives a value of 721 kJ to raise 1 kg of water from 0°C to its saturation temperature of 170°C.

The heat energy (enthalpy of evaporation) needed by the water at 7 bar g to change it into steam is actually less than the heat energy required at atmospheric pressure. This is because the specific enthalpy of evaporation decreases as the steam pressure increases.

However, as the specific volume also decreases with increasing pressure, the amount of heat energy transferred in the same volume actually increases with steam pressure.
Kaustubh Katdare
Kaustubh Katdare • Jan 21, 2009
@Mangesh - could you please provide the source of the information?
jav
LET YOUR HEART BE IN PEACE

Hi, Sir, i wanna method for increasing the pressure of the steam, can you guide me in what all ways can i do that, I will be glad if you guide me through this.. thank you and eagerly awaiting for your reply.....
by
javeed
Dear Javeed, Refer to universal gas equation i.e.PV=mRT where P is pressure, V is volume and R is a gas constant and T is absolute temperature. It shows that Pressure is inversely propertional to Volume. Thus for a given container size, if the flow of steam if restricted; pressure will go on increasing till the bursting force on container.

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