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You are using an out of date browser. It may not display this or other websites correctly.You should upgrade or use an alternative browser. Steam Tables - g vs. fg

• Thread starter mechgirl
• Start date Apr 6, 2010

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mechgirl

Well-known member

Joined Apr 1, 2008 Messages 48 Reaction score 0 I know this is very basic, but I'm feeling like I'm missing something. Enthalpy as an example: I know that hf is enthalpy of saturated liquid, and hg is enthalpy of saturated vapor, and hfg = (hg - hf). I was working some vapor cycle problems last week, and from the steam tables, I would select hg for saturated steam at the given temp/pressure. Instead of hg, the solution selected hfg. I wish I could remember which problem, but I don't even remember which book I was working in. I just continued working problems at the time, but now I'm thinking about it and feeling confused. So, when would I select "fg" instead of "g" from the steam tables? I just feel so overloaded and tired and frustrated now, and like I'll never pass. A week ago, I was feeling pretty confident. Ugh. Ok, must refocus...

Firefly

Active member

Joined Dec 14, 2009 Messages 42 Reaction score 0 Location Charleston, SC hfg can be used as the latent energy and is used when finding the quality. B

Bman

Well-known member

Joined Jul 3, 2007 Messages 182 Reaction score 0 Location Ft Lauderdale, Fl fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it. M

mechgirl

Well-known member

Joined Apr 1, 2008 Messages 48 Reaction score 0

Bman said: fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it. Click to expand...

I know to use hfg for quality. I think this problem was a vapor power cycle, and I was finding the enthalpy of sat steam at a point. The solution used hfg while I used hg. They weren't finding quality, but said that hfg was the enthalpy of the steam at that point. It didn't make sense to me. What other times, besides finding quality, would I need hfg? Why would I select hfg instead of hg at a sat steam state point?

Baltimore Joe

Joe

Joined Jan 6, 2009 Messages 37 Reaction score 0 Location Lutherville Timonium

mechgirl said:

Bman said: fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it. Click to expand...

I know to use hfg for quality. I think this problem was a vapor power cycle, and I was finding the enthalpy of sat steam at a point. The solution used hfg while I used hg. They weren't finding quality, but said that hfg was the enthalpy of the steam at that point. It didn't make sense to me. What other times, besides finding quality, would I need hfg? Why would I select hfg instead of hg at a sat steam state point? Click to expand...

I’d focus on phase change, specifically at the boiler or condenser. Here are locations when H fg may be applied, provided that the steam is going from a saturated liquid to a saturated gas or visa versa. G

gaidox

Project Engineer

Joined Mar 9, 2010 Messages 69 Reaction score 0 Location California

Baltimore Joe said:

mechgirl said:

Bman said: fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it. Click to expand...

I know to use hfg for quality. I think this problem was a vapor power cycle, and I was finding the enthalpy of sat steam at a point. The solution used hfg while I used hg. They weren't finding quality, but said that hfg was the enthalpy of the steam at that point. It didn't make sense to me. What other times, besides finding quality, would I need hfg? Why would I select hfg instead of hg at a sat steam state point? Click to expand...

I’d focus on phase change, specifically at the boiler or condenser. Here are locations when H fg may be applied, provided that the steam is going from a saturated liquid to a saturated gas or visa versa. Click to expand...

Agree with them.. In addition to this, when things were little confusing with thermo phases, I recommend refreshing the rules for determining phases (merm 24.2), latent heats (merm 24.16), and thermo. properties (merm 24.25~30). These are good basic stuffs for thermo. I've noticed in samples exams that a lot of problems relies on the basics and will definitely helpful (to really master these) come exam day. I keep refreshing these when I got confused. -37% $49.99 $79.95 Calculated Industries 4065 Construction Master Pro Advanced Construction Math Feet-inch-Fraction Calculator for Contractors, Estimators, Builders, Framers, Remodelers, Renovators and Carpenters Amazon.com $116.95 ENGINEER Handy Crimp PAD-02 Tool Kit' - Versatile Precision Universal Crimping Set with Inter-Changeable Dies.Made in Japan.,Orange Bridge Precision Tools $21.99 Mr. Pen- Architect Set, Professional Geometry Set, 17 Pcs, Compass and Protractor Set, Drafting Tools, Scale Ruler, Drawing Stencils, Metal Ruler Set, Christmas Gift Mr. Pen $13.99 KETIPED 3D Multi-Angle Measuring Ruler,45/90 Degree Aluminum Alloy Woodworking Square Protractor,Drawing Line Ruler, Miter Triangle Ruler High Precision Layout Measuring Tool for Engineer Carpenter,R MojiaTrading -20% $79.97 $99.99 KNINE OUTDOORS Precision 1/4" Torque Screwdriver Set Adjustable from 10 to 70 In-Lbs Engineering Repair Kit Maintenance Tools with T-BAR Ratchet Screwdriver, Set of 92 pieces Knine Outdoors -40% $17.99 $29.99 Casio fx-115ESPLUS2 2nd Edition, Advanced Scientific Calculator Amazon.com -46% $46.00 $84.99 Casio fx-9750GIII, Standard Graphing Calculator, Python and Natural Text Book Display, Black 17Photo, Electronics and Outdoors -11% $24.99 $27.99 Casio FX-991ES Plus-2nd Edition Scientific Calculator The Watch Box $99.99 Automatic Level 32X Optical Transit Survey Auto Level High Precision Level Gauge Measure Meter Machine Yae First Trading -20% $26.39 $32.99 Scientific Calculators, IPepul 991EX Type-C Rechargeable 552 Functions Desktop Math Calculator with Notepad,5-Lines Display Classroom Must Haves School Supplies for Middle High School/College Students Woot $42.99 RENUS 8 Packs, 2-Line Engineering Scientific Calculator Function Calculator for Student and Teacher 16 AAA Batteries Included RenusDirect -38% $27.22 $43.99 Wheeler Engineering 45-Piece SAE/Metric Hex and Torx Key Set with Storage Case for Gunsmithing and Firearm Maintenance Amazon.com $19.98 ESTWING Sure Strike Blacksmith's Hammer - 40 oz Metalworking Tool with Fiberglass Handle & No-Slip Cushion Grip - MRF40BS Amazon.com -9% $19.97 $21.97 THORVALD 6-in-1 Carpenter Square 7 inch (1/16" Precision) T Square Multi-Purpose Carpenter Tools : Wrench, Square, Protractor, Ruler, Marking Gauge, Drill Gauge, Includes 2-in-1 Carpentry Pencil THORVALD U.S.A. $54.97 KNINE OUTDOORS Screwdriver Set Engineering Repair Kit Maintenance Tools Durable Construction Accessories with Ratcheting Wrench Rubber Mallet Bits Wrenches Adapters Pin Punches, 100 Set Knine Outdoors -24% $27.31 $35.99 Wheeler Hammer and Punch Set with Punches, Brass/Polymer Hammer, Storage Case for Gunsmithing, Firearm Maintenance, Black Amazon.com -23% $115.98 $150.00 Texas Instruments TI-84 Plus CE Color Graphing Calculator, Black Amazon.com -5% $166.99 $175.00 Texas Instruments TI-Nspire CX II CAS Color Graphing Calculator with Student Software (PC/Mac) Amazon.com Post reply

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