|
ARTICLES
DEFINITIONS
PROPERTIES OF SATURATED WATER AND STEAM
CONVERSION FACTORS
BOILER EFFICIENCIES
Introduction
The efficiency of a boiler should be an important part of a purchase evaluation since the annual cost of fuel can easily be 2 to 3 times the installed cost of the equipment. Therefore, a difference in efficiency and the resultant difference in fuel cost can easily offset a difference in installed cost. In many cases, the fuel savings in the first year alone can exceed a difference in installed cost and, of course, fuel savings are on-going ? year after year, after year.
While it is important to consider efficiency in an equipment purchase, it is equally important to understand efficiency to the point that the purchaser can be assured that values are being compared on an apples-to-apples basis. The subject of efficiency for a boiler is rather complex when all of the elements that affect efficiency are considered and a complete thermodynamic analysis is performed. Fortunately, it is not necessary to understand the process in detail, but a basic understanding of the terms can help ensure a good apples-to-apples efficiency evaluations. These factors are discussed in the context of the discussion on efficiency terms.
Efficiency Terms
There are several terms used to qualify efficiency when used in the context of a boiler. These include, simply efficiency, boiler efficiency, thermal efficiency, combustion efficiency and fuel-to-steam efficiency.
The terms, Efficiency and Boiler Efficiency, by themselves are, essentially, meaningless since they must be qualified in order to understand their significance.
In general, the term, Thermal Efficiency refers to the efficiency of a thermal process. This is as opposed to Mechanical Efficiency ? the efficiency of a mechanical process. When used in conjunction with boilers, Thermal Efficiency sometimes refers to the efficiency of the heat exchanger. In any event, this term is not significant for purposes of comparing one boiler, or steam generator, to another. While the thermal efficiency of the heat exchanger is an important factor, its importance lies in its contribution to the Fuel-to-Steam Efficiency.
While the terms Efficiency and Thermal Efficiency are not meaningful for comparing one boiler to another, the terms Combustion Efficiency and Fuel-to-Steam Efficiency are. Of these, Fuel-to-Steam Efficiency is the most significant but is difficult to measure or calculate in real world situations. Therefore, Combustion Efficiency that can be easily computed using a combustion gas analyzer is, frequently, used for performance comparison purposes.
Combustion Efficiency equals the total heat released in combustion, minus the heat lost in the stack gases, divided by the total heat released. For example, if 1000 BTU/Hr are released in combustion and 180 BTU/Hr are lost in the stack, then the combustion efficiency is 82%: (1000 ? 180)/1000 = 0.82 or 82%.
Fuel-to-Steam efficiency is the most important because it is a measure of the energy that is converted to steam and that is, after all, the reason a user installs a steam boiler ? to produce steam. Fuel-to-Steam efficiency is equal to combustion efficiency less the percent of heat losses through radiation and convection. For example, as in the example above, 20 BTU/Hr are lost to convection and radiation then the convection and radiation losses are 2%: 20/1000 = .02 or 2%. If the combustion efficiency for this same case is 82% then the Fuel-to-Steam efficiency is 80%: 82% - 2% = 80%.
(Note: When comparing efficiencies it is important to know if the efficiency is based on the High Heat Value (HHV) or Low Heat Value (LHV) of the fuel. Both are essentially "correct" but comparing an efficiency based on HHV to one based on LHV would not be correct. In the United States boiler efficiencies are, typically, based on the HHV. In Europe they are, typically, based on the LHV and result in a higher value than when based on HHV. The general relationship is: Efficiency based on LHV = Efficiency based on HHV X 1.11 for natural gas and X 1.06 for diesel fuel oil.)
Operating Efficiency
Each of the terms discussed, above, refer to the efficiency of a boiler when operating at a fixed condition. For instance, at 100% load, with specified air and feedwater temperatures, etc. These efficiencies are, unquestionably, important but there are operational factors that affect the annual fuel bill and can have an affect that may be greater than the difference of a point or two in the efficiency of the equipment when, for instance, operating at 100%. These factors are discussed on the Fuel Savings page.
The Boiler Blowdown Considerations article provides further information on the topic of blowdown and how it can affect operating efficiency.
STEAMING RATE
The Steaming Rate (the rate at which a boiler produces steam, normally expressed in terms of Lbs/Hr or Kg/Hr) is an item that is frequently misunderstood and such a misunderstanding can lead to the purchase of the wrong size boiler. It is, therefore, essential that the Steaming Rate be qualified when selecting a boiler size.
The three common Steaming Rate terms are:
- From and at 212 ºF (100 °C)Steaming Rate
- Gross Steaming Rate
- Net Steaming Rate
From and at 212 ºF (100 °C) is the Steaming Rate for a boiler producing steam, at the outlet flange, at 212 ºF (100 °C), and 0 PSIG, with feedwater at the inlet flange at 212 ºF (100 °C) and 0 PSIG. This is the most common steaming rate term and is used most often in brochures, etc. when steaming rate information is provided. One Boiler Horsepower (BHP) is, by definition, equivalent to 34.5 pounds of steam per hour, from and at 212 ºF (100 °C).
Gross Steaming Rate is the rate at which a boiler produces steam, at the outlet flange, based on application specific feedwater conditions at the inlet flange and application specific steam conditions. The Gross Steaming Rate, typically, differs from the From and at 212 ºF (100 °C) Steaming Rate because both the feedwater inlet and the steam conditions are different than 212 ºF (100 °C) and 0 PSIG.
A typical application may, for instance, have feedwater at 190 ºF and produce saturated steam at 100 PSIG (338 ºF). Since the inlet temperature is less than 212 ºF (100 °C) and the outlet temperature is greater than 212 ºF (100 °C), the amount of heat needed to produce a pound of steam, at these conditions, is greater than the amount needed to produce a pound of steam with inlet and outlet temperatures of 212 ºF (100 °C). The Gross Steaming Rate is, therefore, frequently, less that the From and at 212 ºF (100 °C). It may, however, actually be greater, if the feedwater receiver is a pressurized deaerator that heats the feedwater to a temperature above 212 ºF (100 °C), for instance, 230 ºF.
Net Steaming Rate is the steaming rate at which a boiler produces steam, to your plant or process and, thus, is the most important steaming rate to consider. Net steaming rate, differs from Gross Steaming Rate in that it takes into account the amount of steam needed to heat the feedwater in the feedwater receiver (deaerator or hotwell): specifically, the Net Steaming Rate equals the Gross Steaming Rate minus the steaming rate to the feedwater receiver. Except for some very unusual applications, the Net Steaming Rate is less than the Gross or From and at 212 ºF (100 °C) Steaming Rate.
Take, for example, a 100 BHP boiler operating with 100% make-up water at 60 ºF and producing steam at 125 PSIG. In this case, the From and at 212 ºF (100 °C) Steaming rate is 3,450 Lbs/Hr but the Net Steaming Rate is only 2874 Lbs/hr - 17% less than the From and at 212 ºF (100 °C) Steaming Rate.
The effect of feedwater heating is applicable in all applications and, thus, should always be considered. There is another factor that has an effect and can be significant in some applications. That factor is the amount of "blowdown" that is required in order for the boiler to operate effectively. In this case, "blowdown", refers to the amount of water that must be removed from the boiler system, on a regular basis, in order to control the level of Total Dissolved Solids (TDS) in the boiler. Water that is removed to control TDS has been heated and the amount of energy needed to heat this water reduces the amount of energy that is available to produce steam. (See the Blowdown Considerations article).
In summary, users should be certain to qualify steaming rates when using them to define the size of a boiler. Boiler Horsepower is a specific term and no further information is needed to select the size of a boiler. However, if a steaming rate is used to specify boiler size then the steaming rate must be qualified - From and at 212 ºF (100 °C), Gross, or Net pounds, or KG, per hour.
BOILER BLOWDOWN CONSIDERATONS
Boilers require periodic blowdown in order to maintain effective operation, provide for good equipment life, and reduce maintenance time and expense. "Blowdown? refers to the removal of boiler water in order to maintain an acceptable level of Total Dissolved Solids (TDS). Blowdown has an economic impact because the water that is removed has been heated and chemically treated and the energy used to heat this water comes from the fuel burned in the boiler. In most cases, blowdown with a Clayton Steam Generator is significantly less than with conventional boilers and this reduction results in significant fuel savings.
Blowing down is the process of removing boiler water that has a maximum acceptable level of concentration. The water that is blown down is replaced by make-up water that has a much lower TDS level. This dilutes, or lowers, the concentration in the boiler water. The higher the TDS level in the blow down water the lower the amount of water that must be removed.
The amount of water that must be blown down for any given application depends upon:
- The TDS level in the make-up water ? the higher the level the greater the amount of blowdown.
- The amount of make-up water vs. the amount of condensate returned ? the greater the percent of make-up the greater the amount of blowdown.
- The maximum acceptable TDS level in the boiler water ? the lower the level the greater the amount of blowdown.
- The TDS in the blowdown water ? the higher the TDS, the lower the amount of blowdown.
- The average load level for the boiler (BHP or lbs. of steam per hour).
As can be seen from the above, applications with high make-up levels, high TDS in the make-up water and extended operation at high load levels result in high levels of blowdown.
The TDS of the blowdown water is significant because the higher the TDS, the lower the volume of water that must be removed.
Clayton Steam Generators provide a fuel savings, from reduced blowdown, because of two factors:
- First, since the Steam Generator is a forced circulation boiler it can tolerate relatively high TDS levels in the feedwater ? as high as 8550 ppm.
- Second, water that is blown down is separator trap return water that has been concentrated to 4 to 5 times that of the feedwater level.
(See How We Make Steam for an explanation of the Clayton system and trap return water.)
Considering these two factors, blowdown water for a Clayton Steam Generator, typically, has a concentration of 24,000 to 40,000 ppm. That compares to values for conventional boilers in the range of 2,500 to 3,500 ppm. Since our blowdown water has TDS concentrations that are 7 to 16 times higher than conventional boilers, the volume of blowdown water with Clayton is 1/16 to 1/7 that of a conventional boiler. As will be shown, this can amount to a major savings in fuel cost.
In addition to the factors that determine the volume of blowdown water, the economic impact, depends upon:
- The temperature of the condensate and make-up water ? the lower the temperatures the greater the economic impact.
- The steam pressure ? the higher the pressure, and thus the temperature, the greater the economic impact.
- The cost of fuel ? obviously, the higher the fuel cost the greater the economic impact.
- The number of hours of operation.
- The efficiency of the boiler.
- The cost of chemicals.
- The cost of water.
- The cost of disposal of blowdown water.
These savings can be illustrated by the following example:
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Make-up Water TDS Level
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300 ppm
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Make-up water temperature:
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60 Degrees F
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% of Make-up:
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100%
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Blowdown TDS level in conventional boiler:
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3000 ppm
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TDS level in Clayton feedwater:
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6500 ppm
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Average boiler load:
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300 BHP
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Boiler Efficiency:
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80%
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Steam Pressure
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125 PSIG
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|
Hours of operation per day:
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16 hours per day
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Days of operation per year:
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312 days per year
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Cost of natural gas:
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$4.00 per MMBTU
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In this case, the savings in blowdown, is 860 lbs. of water per hour, which equates to annual savings, based on the assumptions shown, of nearly $6,400. This could easily represent greater than 6% of the installed cost of the new equipment and, most importantly, these savings are ongoing.
As noted, savings with a Clayton Steam Generator that result from reduced blowdown can be significant. These savings are application dependent. Please contact us if you would like an estimate for your application.
DEFINITIONS
Boiler Horsepower (BHP)- The amount of energy needed to produce 34.5 pounds of steam, per hour, at a pressure and temperature of 0 Psig and 212 ºF, with feedwater at 0 Psig and 212 ºF. This is equivalent to 33,475 BTU/Hr or 8430 Kcal/Hr.
Calorie (C) - The amount of heat required, at a pressure of one atmosphere, to raise the temperature of one gram of water one degree Celsius.
Enthalpy (H or h) - Thermodynamically, Enthalpy is defined as the sum of the internal energy of a body and the product of its volume multiplied by its pressure. For the sake of boiler calculations, Enthalpy can be defined as the amount of heat in a fluid, usually expressed as BTU/Lb or Kcal/Gram. In these cases, Enthalpy is considered to be zero at 32 ºF (0 ºC).
From and at 212 ºF - A term used to qualify the amount of steam produced by a boiler (see the article on Steaming Rate). The qualification indicates that the amount of steam produced is at a pressure of 0 Psig and 212 ºF, with feedwater at 0 Psig and 212 ºF.
Gross Steaming Rate - the amount of steam produced by a boiler at the outlet flange of the boiler. This is the amount of steam produced before steam is removed to heat the water in the feedwater receiver (deaerator or hotwell).
Heat of Vaporization - (for boiler calculation purposes) the amount of heat required to convert water, at saturated conditions, to vapor (steam) at the same saturated conditions. Same as Latent Heat.
Latent Heat - See Heat of Vaporization.
Net Steaming Rate - The amount of steam produced, by a boiler, after blow-down and after steam is used for heating the water in the feedwater receiver (dearator or hotwell).
Saturated Liquid - Liquid that is at a Saturated Pressure at Temperature.
Saturated Steam - Steam (Vapor) that is at a Saturated Pressure and Temperature.
Saturated Vapor - Vapor that is at a Saturated Pressure and Temperature.Saturated Water - See Saturated Liquid.
Saturation Pressure - The pressure at which saturation takes place at a given temperature.
Saturation Temperature
Sensible Heat - The heat (Enthalpy), usually expressed as BTU/Lb or Cal/G, in a liquid.
Specific Heat - The amount of heat (Enthalpy) required to raise the temperature of one unit of mass, one degree. Usually expressed as BTU/LB/ºF or Cal/Kg/ºC.
Steaming Rate - The rate, usually expressed in Lbs/Hr or KG/Hr, at which a boiler produces steam. (See the Steaming Rate article.)
Sub-cooled Liquid - A liquid that is at a temperature or pressure below the saturation temperature and pressure.
Superheat - The extra heat imparted to a vapor (steam)in heating it from a dry saturated condition. Also the corresponding rise in temperature.
PROPERTIES OF SATURATED WATER AND STEAM
| Saturated |
Enthalpy |
Specific Volume |
| Conditions |
BTU/LB |
Cu. Ft. / Lb |
| Pressure |
Temp |
Water |
Vaporization |
Vapor |
Water |
Vapor |
| PSIG |
Deg F |
Hf |
Hfg |
Hg |
Vf |
Vg |
| 0 |
212.0 |
180.3 |
970.0 |
1150.3 |
0.01672 |
26.7990 |
| 5 |
227.2 |
195.6 |
960.5 |
1156.1 |
0.01683 |
20.8300 |
| 10 |
239.4 |
208.0 |
952.6 |
1160.6 |
0.01692 |
16.4910 |
| 15 |
249.8 |
218.5 |
945.7 |
1164.2 |
0.01700 |
13.8780 |
| 20 |
258.8 |
227.6 |
939.7 |
1167.3 |
0.01708 |
11.9660 |
| 30 |
274.0 |
243.1 |
929.1 |
1172.2 |
0.01721 |
9.4597 |
| 40 |
286.7 |
256.1 |
919.9 |
1176.0 |
0.01733 |
7.8261 |
| 50 |
297.7 |
267.3 |
911.9 |
1179.2 |
0.01743 |
6.6830 |
| 60 |
307.3 |
277.3 |
904.6 |
1181.9 |
0.01753 |
5.8369 |
| 70 |
316.0 |
286.2 |
897.9 |
1184.1 |
0.01761 |
5.1845 |
| 80 |
323.9 |
294.4 |
891.7 |
1186.1 |
0.01770 |
4.6656 |
| 90 |
331.2 |
301.9 |
885.9 |
1187.8 |
0.01778 |
4.2326 |
| 100 |
337.9 |
308.9 |
880.5 |
1189.4 |
0.01785 |
3.8911 |
| 110 |
344.2 |
315.5 |
875.3 |
1190.8 |
0.01792 |
3.5941 |
| 120 |
350.0 |
321.7 |
870.3 |
1192.0 |
0.01799 |
3.3397 |
| 130 |
355.6 |
327.5 |
865.7 |
1193.2 |
0.01806 |
3.1193 |
| 140 |
360.9 |
333.1 |
861.1 |
1194.2 |
0.01812 |
2.9266 |
| 150 |
365.9 |
338.4 |
856.7 |
1195.1 |
0.01816 |
2.7563 |
| 160 |
370.6 |
343.4 |
852.6 |
1196.0 |
0.01824 |
2.6050 |
| 170 |
375.2 |
348.3 |
848.5 |
1196.8 |
0.01830 |
2.4694 |
| 180 |
379.6 |
353.0 |
844.5 |
1197.5 |
0.01835 |
2.3473 |
| 190 |
383.8 |
357.5 |
841.5 |
1198.9 |
0.08141 |
2.2368 |
| 200 |
387.8 |
361.8 |
837.6 |
1199.4 |
0.01847 |
2.1361 |
| 210 |
391.7 |
366.0 |
834.0 |
1200.0 |
0.01852 |
2.0442 |
| 220 |
395.4 |
370.1 |
830.5 |
1200.5 |
0.01857 |
1.9598 |
| 230 |
399.1 |
374.0 |
826.9 |
1200.9 |
0.01863 |
1.8820 |
| 240 |
402.6 |
377.8 |
823.5 |
1201.3 |
0.01868 |
1.8102 |
| 250 |
406.0 |
381.5 |
820.2 |
1201.7 |
0.01873 |
1.7436 |
| 260 |
409.4 |
385.2 |
816.9 |
1202.1 |
0.01878 |
1.6816 |
| 270 |
412.6 |
388.7 |
813.7 |
1202.4 |
0.01882 |
1.6239 |
| 280 |
415.7 |
392.1 |
810.6 |
1202.7 |
0.01887 |
1.5700 |
| 290 |
418.8 |
395.5 |
807.5 |
1203.0 |
0.01892 |
1.5194 |
| 300 |
421.8 |
398.8 |
804.4 |
1203.2 |
0.01896 |
1.4720 |
| 310 |
424.7 |
402.0 |
801.4 |
1203.4 |
0.01901 |
1.4274 |
| 320 |
427.5 |
405.2 |
798.5 |
1203.6 |
0.01905 |
1.3854 |
| 330 |
430.3 |
408.2 |
795.6 |
1203.8 |
0.01910 |
1.3457 |
| 340 |
433.0 |
411.3 |
792.7 |
1204.0 |
0.01914 |
1.3084 |
| 350 |
435.7 |
414.2 |
789.9 |
1204.1 |
0.01919 |
1.2729 |
| 360 |
438.3 |
417.1 |
787.1 |
1204.2 |
0.01923 |
1.2392 |
| 370 |
440.8 |
420.0 |
784.3 |
1204.3 |
1.01927 |
1.2071 |
| 380 |
443.3 |
422.8 |
781.6 |
1204.4 |
0.01931 |
1.1767 |
| 390 |
445.7 |
425.5 |
778.7 |
1204.2 |
0.01936 |
1.1477 |
| 400 |
448.2 |
427.9 |
776.3 |
1204.2 |
0.01940 |
1.1200 |
| 410 |
450.5 |
430.6 |
773.7 |
1204.3 |
0.01944 |
1.0936 |
| 420 |
452.8 |
433.2 |
771.1 |
1204.3 |
0.01948 |
1.0684 |
| 430 |
455.1 |
435.8 |
768.5 |
1204.3 |
0.01985 |
1.0443 |
| 440 |
457.3 |
438.3 |
766.0 |
1204.3 |
0.01956 |
1.0212 |
| 450 |
459.5 |
440.8 |
763.5 |
1204.3 |
0.01961 |
0.9990 |
| 500 |
470.0 |
452.8 |
751.3 |
1204.1 |
0.01970 |
0.9278 |
| 600 |
488.8 |
474.6 |
728.3 |
1202.9 |
0.02010 |
0.7698 |
| 700 |
503.2 |
491.9 |
708.5 |
1200.4 |
0.02050 |
0.6554 |
| 800 |
517.6 |
509.3 |
688.6 |
1197.9 |
0.02090 |
0.5687 |
| 900 |
532.0 |
526.6 |
668.8 |
1195.4 |
0.02120 |
0.5006 |
| 1000 |
544.6 |
542.4 |
649.4 |
1191.8 |
0.02160 |
0.4456 |
CONVERSION FACTORS
| Atmospheres |
Bars |
1.01325 |
| |
Pounds-force/sq in |
14.696 |
| Boiler Horsepower (BHP) |
BTU/hr |
33,475 |
| |
Kcal/hr |
8435.6 |
| |
Kw |
9.9805 |
| |
Horsepower (mech) |
13.155 |
| BTU |
Cal, g |
251.996 |
| |
Foot pound-force |
777.649 |
| |
Kg-force meters |
107.514 |
| |
Kw-Hours |
0.000292875 |
| BTT/Hr |
Calorie, kg/gram |
0.5555 |
| |
Foot pound-force/lb |
777.649 |
| Calories, g |
BTU/hr |
0.003.971 |
| |
Foot pound-force |
3.08596 |
| |
Gram-force cm |
42664.9 |
| |
Joules |
4.184 |
| |
Kg-force meters |
0.42665 |
| Centimeters |
Inches |
0.3937 |
| Cm of HG O deg C |
Atmospheres |
0.01316 |
| |
Bars |
0.01333 |
| |
pound-force/sq in |
0.1934 |
| Cu Centimeters |
Cu ft |
3.51E-05 |
| |
Cu inches |
0.06102 |
| Cu Feet |
Cu centimeters |
28316.8 |
| |
Cu meters |
0.02832 |
| Cu ft/pound |
Cu cm/gram |
92.428 |
| Cu ft/sec |
Cu cm/sec |
28316.8 |
| |
Liters/min |
1699.01 |
| Cu feet |
Cu meters |
35.3147 |
| Cu feet of water at 32 deg F |
Pounds |
62.4 |
| Degrees Centigrade |
Degrees Fahrenheit |
F = [C X (1/8)] + 32 |
| Degrees Fahrenheit |
Degrees Centigrade |
C= (F-32) X 0.555 |
| Dynes |
Gram-force |
0.0010197 |
| |
Joules/meter |
1.00E-06 |
| |
Pound force |
2.25E-06 |
| Feet |
Centimeters |
30.48 |
| |
Meters |
0.3048 |
| Gallons (Brit) |
Gallons (US liq) |
1.2009 |
| |
Liters/min |
4.546 |
| Gallons (US liq) |
Gallons (Brit) |
0.83267 |
| Gallons (US) of water at 60 deg F |
Pounds |
8.337 |
| Grains of hardness per gallon |
Parts/million |
17.117 |
| Inches of water at 39.2 deg F |
Pound-force/sq in |
0.03613 |
| Inches |
Centimeters |
2.54 |
| Kilograms |
Pounds (avdp) |
2.2046 |
| Kilogram-force/sq cm |
Pounds-force/sq in |
14.223 |
| Kilowatts |
BTU/Hr |
3414.4 |
| Liters |
Cu cm |
1000 |
| |
Cu feet |
0.03531 |
| |
Cubic inches |
61.0237 |
| |
Gallons (US liq) |
0.26417 |
| Meters |
Feet |
3.28094 |
| |
Inches |
39.3701 |
| |
Yards |
1.0936 |
| Parts/million |
Grains/Gal (US liq) |
0.05842 |
| Pound-force/sq in |
Atmospheres |
0.06805 |
| |
Bars |
0.06895 |
| |
Kg-force/sq cm |
0.07031 |
| Pounds of water at 32 deg F |
Cu feet |
0.01602 |
| Tons (metric) |
Kilograms |
1000 |
| |
Pounds (advp) |
2205 |
| Tonnes |
Kilograms |
1000 |
| Tons-refrigeration |
BTU/hr |
12000 |
| Watts |
BTU/hr |
3.41214 |
| Zoll (Switzerland) |
Centimeters |
3 |
|
