Energy, Heat and Temperature: The Units a Boiler Engineer Uses
Heat is energy on the move, and temperature is how hot something is. They are measured in different units, and an Indian boiler house uses several of each. This lesson sorts them out, so that every later lesson can use them without stopping.
You'll learn
- Tell energy, heat and temperature apart
- Convert between kJ, kcal and kWh, and between kW and lakh kcal/h
- Convert between Celsius, kelvin and Fahrenheit
- Know when a calculation needs absolute temperature
Energy, heat and temperature are not the same thing
Energy is the capacity to do work or to warm something. Heat is energy on its way from a hotter body to a cooler one: from a flame into a boiler tube, from steam into a product. Temperature is not an amount of anything; it is a measure of how hot a body is, and it decides which way heat will flow.
A bucket of water at 60 °C and a swimming pool at 30 °C show the difference. The bucket is hotter, so heat would flow from it into the pool. But the pool holds far more heat, because there is so much more of it. Boiler work constantly needs both ideas: how hot (temperature) and how much (heat).
The units you will meet on an Indian plant
| Quantity | Unit | Where you see it |
|---|---|---|
| Energy, heat | kJ (kilojoule) | Steam tables, engineering calculations |
| Energy, heat | kcal (kilocalorie) | Fuel calorific values, older steam tables |
| Energy | kWh | Electricity bills, electric boilers |
| Heat rate (power) | kW | Heat exchangers, electric heaters |
| Heat rate (power) | lakh kcal/h | Thermic fluid heaters, hot water generators |
| Steam output | TPH or kg/h | Boiler ratings |
| Temperature | °C | Every gauge on the plant |
| Temperature | K (kelvin) | Physics and some formulas |
Notice that heat and heat rate are different. A kJ is an amount; a kW is an amount per second. A heater rated in lakh kcal/h is quoting a rate, the same kind of quantity as kW.
Converting heat and power
So 1 kWh is 860 kcal. Power follows the same factors: 1 kW is 1 kJ every second, so 1 kW = 3,600 kJ/h = 860 kcal/h. A lakh is 100,000.
- Rated output: 6 lakh kcal/h = 6,00,000 kcal/h.
- Convert kcal/h to kJ/h, then divide by 3,600 seconds per hour:
Converting temperature
Celsius and kelvin have the same step size: a rise of 1 °C is a rise of 1 K. Only their zero points differ. Fahrenheit steps are smaller, 1.8 of them to each degree Celsius. Imported equipment data sheets and older American references quote °F; convert before using them.
| What | °C | K | °F |
|---|---|---|---|
| Water freezes | 0 | 273.15 | 32 |
| Typical make-up water | 30 | 303.15 | 86 |
| Water boils at sea level | 100 | 373.15 | 212 |
| Steam at 10 kg/cm²(g) | 183.3 | 456.5 | 362.0 |
When the zero point matters
Any formula built on a temperature difference works in °C or K equally. Any formula built on a temperature itself must use kelvin. Three you will meet: radiant heat, which goes with the fourth power of absolute temperature (so a furnace at 1,000 °C radiates far more than twice as much as one at 500 °C); the gas laws, used for air and flue gas volumes; and the Carnot limit on how much heat any engine can turn into work.
The kilocalorie also has a trap. There are two definitions in circulation: the International Table calorie, 4.1868 J, used in engineering and in Indian boiler practice, and the thermochemical calorie, 4.184 J, used in chemistry. The difference is under 0.1%, which is invisible in a fuel calculation but enough to make two careful engineers’ figures disagree in the last digit.
Key takeaways
- Temperature says how hot; heat says how much energy moves. They are different quantities.
- 1 kcal = 4.1868 kJ; 1 kWh = 3,600 kJ; 1 lakh kcal/h ≈ 116 kW.
- K = °C + 273.15; a temperature difference is the same number in both.
- Use kelvin whenever a formula uses temperature itself rather than a difference.