Steam is the most versatile heat transfer medium in industrial plants, and steam tables are the reference that makes every steam system calculation possible. Whether you are sizing a steam trap, calculating boiler efficiency, estimating flash steam recovery potential, or verifying deaerator performance, the answer starts with looking up the properties of water and steam at operating conditions.
This guide covers how to read and use saturated and superheated steam tables, the key calculations every plant operator and stationary engineer needs, and the common mistakes that lead to wasted energy, failed steam traps, and water hammer.
Reading Steam Tables: The Essential Properties
Saturated steam tables are organized by pressure (or temperature) and provide the properties of water and steam at the point where liquid and vapor coexist in equilibrium. The key properties are:
Saturation temperature (Tsat): The boiling point of water at the given pressure. At 100 PSIG (114.7 PSIA), Tsat is 338\x{00B0}F. At 15 PSIG (29.7 PSIA), Tsat is 250\x{00B0}F. Higher pressure = higher boiling point. This is the foundation of every steam system.
Enthalpy of liquid (hf): The heat content of saturated water in BTU/lb. At 100 PSIG, hf = 309 BTU/lb. This is the energy already in the water at the boiling point.
Enthalpy of evaporation (hfg): The latent heat required to convert one pound of saturated water to one pound of saturated steam at constant pressure. At 100 PSIG, hfg = 881 BTU/lb. This is the useful heat released when steam condenses in a heat exchanger.
Enthalpy of steam (hg): The total heat content of saturated steam, equal to hf + hfg. At 100 PSIG, hg = 1,190 BTU/lb. This is the energy delivered by each pound of steam from the boiler.
Specific volume (vg): The volume occupied by one pound of saturated steam. At 100 PSIG, vg = 3.89 ft3/lb. At 15 PSIG, vg = 13.7 ft3/lb. Steam expands dramatically as pressure drops, which is why low-pressure steam requires larger pipes.
Steam Trap Sizing from Steam Tables
Steam traps remove condensate from steam systems while preventing live steam from escaping. Proper trap sizing requires knowing the condensate load (lb/hr), the pressure differential across the trap, and a safety factor to handle startup and upset conditions.
The condensate load from a heat exchanger equals the heat duty divided by the latent heat of steam at the operating pressure: m_condensate = Q / hfg. For a heat exchanger removing 500,000 BTU/hr from 100 PSIG steam (hfg = 881 BTU/lb), the condensate rate is 568 lb/hr.
Apply a safety factor of 2x for normal operation and 3x for applications with heavy startup loads (large heat exchangers, steam mains). So the trap must handle 1,136 lb/hr at the available pressure differential.
The pressure differential across the trap is the steam pressure minus the condensate return pressure. If the steam is at 100 PSIG and the condensate returns to a 5 PSIG header, the differential is 95 PSI. The trap capacity increases with differential pressure. Select a trap from the manufacturer's capacity tables at the calculated load and differential.
The most common sizing mistake is forgetting the startup load. A cold heat exchanger absorbs heat from incoming steam to warm the metal mass and the product. During startup, condensate rates can be 3-5 times the steady-state rate. An undersized trap floods during startup, causing water hammer and slow heat-up.
Flash Steam Recovery
When high-pressure condensate is released to a lower pressure, some of the liquid flashes to steam because the condensate contains more heat (enthalpy) than saturated water can hold at the lower pressure. This flash steam is often vented to atmosphere, wasting the energy it contains.
The flash steam percentage is calculated from the steam tables: flash % = (hf_high - hf_low) / hfg_low x 100. For condensate dropping from 150 PSIG (hf = 339 BTU/lb) to 15 PSIG (hf = 250 BTU/lb, hfg = 946 BTU/lb): flash % = (339 - 250) / 946 x 100 = 9.4%.
On a system producing 5,000 lb/hr of 150 PSIG condensate, that is 470 lb/hr of flash steam at 15 PSIG. At 946 BTU/lb of latent heat, that is 445,000 BTU/hr of recoverable energy, equivalent to about 30 therms of natural gas per hour or roughly $3.00/hr at typical gas prices. Over 8,000 operating hours per year, that is $24,000 in wasted energy.
Flash steam recovery systems collect the flash steam in a flash tank and pipe it to low-pressure steam users: deaerators, feedwater heaters, space heating coils, or domestic hot water systems. The economics are straightforward: if you have a low-pressure steam user and a source of high-pressure condensate, a flash tank pays for itself quickly.
Boiler Efficiency from Steam Tables
Boiler efficiency is the ratio of useful heat output to fuel energy input. The useful heat output per pound of steam is the difference between the enthalpy of the steam leaving the boiler (hg or h_superheat) and the enthalpy of the feedwater entering (hf at feedwater temperature).
For a boiler producing 100 PSIG saturated steam (hg = 1,190 BTU/lb) with feedwater at 200\x{00B0}F (hf = 168 BTU/lb): useful heat = 1,190 - 168 = 1,022 BTU/lb. If the boiler burns 8.5 therms of gas per 1,000 lb of steam, the fuel input is 850,000 BTU per 1,000 lb. The useful output is 1,022,000 BTU per 1,000 lb. Efficiency = 1,022,000 / 850,000 = 120%. That is obviously wrong, which means the fuel input measurement is wrong or the feedwater temperature measurement is off.
The input-output method requires accurate fuel flow measurement, fuel heating value, steam flow measurement, and feedwater temperature. The heat loss method (ASME PTC 4) is often more practical: measure the losses (stack loss, blowdown loss, radiation loss, unburned fuel) and subtract from 100%.
Stack loss is the largest single loss in most boilers, typically 15-25% of fuel input. It is calculated from the stack temperature and excess air percentage using the steam tables and combustion equations. Lowering stack temperature by 40\x{00B0}F typically improves efficiency by 1%. Reducing excess air from 30% to 15% improves efficiency by 1-2%.
Deaerator Operation and Verification
Deaerators remove dissolved oxygen from boiler feedwater by heating the water to saturation temperature at the deaerator operating pressure. At saturation, water cannot hold dissolved gases, and the oxygen is vented to atmosphere.
A deaerator operating at 5 PSIG should heat feedwater to the saturation temperature at 5 PSIG, which is 228\x{00B0}F per the steam tables. If your deaerator outlet temperature is 215\x{00B0}F, the unit is not reaching saturation and dissolved oxygen is not being fully removed. This indicates insufficient steam supply, a malfunctioning vent condenser, or excessive cold water makeup flow overwhelming the heating capacity.
Verify deaerator performance by measuring outlet temperature and comparing to the saturation temperature at operating pressure. Also measure dissolved oxygen: the target is less than 7 ppb (parts per billion) for high-pressure boiler feedwater. Values above 20 ppb indicate inadequate deaeration and will accelerate oxygen pitting corrosion in the boiler.
The steam consumption of a deaerator equals the enthalpy rise of the feedwater divided by the latent heat of steam at the deaerator pressure. If 10,000 lb/hr of makeup water enters at 60\x{00B0}F (hf = 28 BTU/lb) and must be heated to 228\x{00B0}F (hf = 196 BTU/lb) at 5 PSIG (hfg = 960 BTU/lb): steam required = 10,000 x (196 - 28) / 960 = 1,750 lb/hr.
When and Why Superheated Steam Is Used
Superheated steam is steam heated above its saturation temperature at a given pressure. At 100 PSIG, saturated steam is 338\x{00B0}F. If you heat it further to 500\x{00B0}F at the same pressure, it is superheated by 162\x{00B0}F.
Superheated steam is used primarily for power generation (turbines) and long-distance steam distribution. Turbines benefit from superheat because the additional energy increases the available work per pound of steam and keeps the steam dry longer through the turbine stages, reducing blade erosion from water droplets.
For process heating, saturated steam is almost always preferred. Saturated steam condenses at a constant temperature, providing a uniform heat transfer rate. Superheated steam must first cool to saturation before it can condense, and the desuperheating stage has poor heat transfer because it is gas-phase (convective) rather than condensing (very high coefficient). A heat exchanger receiving superheated steam often performs worse than one receiving saturated steam because the desuperheating zone transfers heat slowly.
Superheated steam tables are organized by pressure and temperature. At each pressure-temperature combination, the table provides enthalpy, specific volume, and entropy. The enthalpy of superheated steam at 100 PSIG and 500\x{00B0}F is about 1,279 BTU/lb, compared to 1,190 BTU/lb for saturated steam at the same pressure. The difference (89 BTU/lb) is the sensible superheat energy, which is only about 10% of the total enthalpy. This is why desuperheating is a small fraction of the total heat exchange.