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The energy saving calculation of waste heat recovery from exhaust gas involves the evaluation of heat recovery amount and energy saving benefits. The following are commonly used calculation formulas and explanations:
1. Calculation formula for waste heat recovery from exhaust gas
The heat (Q) of exhaust gas waste heat recovery can be calculated by the following formula:
Q = m · c · ΔT- Q: Recovered heat (unit: kJ or kcal)
- m: Mass flow rate of exhaust gas (unit: kg/s or kg/h)
- c: Specific heat capacity of exhaust gas (unit: kJ/(kg·℃), generally dry air is about 1.0, moisture needs to be adjusted according to the composition)
- ΔT: The temperature drop of the exhaust gas in the waste heat recovery equipment (such as the heat exchanger) (unit: °C), that is, the inlet and outlet temperature difference
Note: If the exhaust gas flow is given as volume flow (m³/s or m³/h), it needs to be converted to mass flow through density (ρ):
m = V · ρAmong them, ρ is determined according to the exhaust gas temperature and composition (for example, at normal pressure, the density of air at 100°C is about 0.946 kg/m³).
2. Heat exchanger efficiency considerations
The actual recovered heat is affected by the heat exchanger efficiency (η), and the correction formula is:
Qactual = η · Q- η: Heat exchanger thermal efficiency (usually 0.6-0.9, depending on equipment design and operating conditions)
- Q: Theoretical maximum heat recovery
3. Energy saving benefit calculation
(1) Fuel saving
If waste heat is used to replace fuel (such as natural gas, coal), the amount of fuel saved is:
B = Qactual / ( Qfuel · ηboiler )- B: Amount of fuel saved (unit: kg or m³)
- Qfuel: The lower calorific value of the fuel (unit: kJ/kg or kJ/m³, such as natural gas about 35,000 kJ/m³)
- ηboiler: Combustion efficiency of boiler or heating equipment (usually 0.8-0.95)
(2) Save energy costs
The economic benefits of energy saving are:
C = B · P- C: Cost savings (unit: yuan)
- P：Fuel unit price (unit: yuan/kg or yuan/m³)
4. Example Calculation
Assume that the exhaust gas flow rate of a factory is 10,000 m³/h, the temperature drops from 300℃ to 100℃, the exhaust gas density is 0.946 kg/m³, the specific heat capacity is 1.0 kJ/(kg·℃), the heat exchanger efficiency is 0.8, the calorific value of natural gas is 35,000 kJ/m³, the boiler efficiency is 0.9, and the unit price of natural gas is 3 yuan/m³.
step:
- Mass flow rate:
m = 10,000 · 0.946 = 9,460 kg/h - Theoretical heat recovery:
Q = 9,460 · 1.0 · (300 - 100) = 1,892,000 kJ/h - Actual heat recovery:
Qactual = 0.8 · 1,892,000 = 1,513,600 kJ/h - Fuel saving:
B = 1,513,600 / (35,000 · 0.9) = 48.02 m³/h - Cost savings:
C = 48.02 · 3 = 144.06 yuan/h
result: Save 48.02 m³ of natural gas per hour and save 144.06 yuan in costs.
5. Notes
- Exhaust gas composition: Exhaust gas containing water vapor or corrosive gas requires adjustment of specific heat capacity and heat exchanger material.
- Heat exchanger type: Common waste heat recovery equipment (such as heat pipe heat exchangers and plate heat exchangers) have different efficiencies and need to be selected based on actual parameters.
- Operating conditions：The exhaust gas flow and temperature may change with production and need to be averaged or dynamically monitored.
- Unit conversion: Ensure that units are consistent (e.g. kJ, kg, °C) and convert when necessary (e.g. 1 kcal = 4.184 kJ).
If you need further calculation or optimization for a specific case (such as waste heat recovery from sludge drying exhaust gas), please provide detailed parameters (such as exhaust gas flow, temperature, composition, etc.)!
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