Exhaust gas waste heat recovery energy saving calculation formula

Table of Contents

• 废气余热回收的热量（Q）可通过以下公式计算： Q = m · c · ΔT Q：回收的热量（单位：kJ 或 kcal） m：废气的质量流量（单位：kg/s 或 kg/h） c：废气的比热容（单位：kJ/(kg·℃)，一般干空气约1.0，湿气需根据成分调整） ΔT：废气在余热回收设备（如换热器）中的温降（单位：℃），即进出口温度差 注：若废气流量以体积流量（m³/s 或 m³/h）给出，需通过密度（ρ）转换为质量流量： m = V · ρ 其中，ρ 根据废气温度和成分确定（如常压下，空气在100℃时密度约0.946 kg/m³）。
• 实际回收热量受换热器效率（η）影响，修正公式为： Q实际 = η · Q η：换热器热效率（通常0.6-0.9，取决于设备设计和运行状况） Q：理论最大回收热量
• （1）节约燃料量 若余热用于替代燃料（如天然气、煤），节约的燃料量为： B = Q实际 / (Q燃料 · η锅炉) B：节约的燃料量（单位：kg 或 m³） Q燃料：燃料的低位发热量（单位：kJ/kg 或 kJ/m³，如天然气约35,000 kJ/m³） η锅炉：锅炉或加热设备的燃烧效率（通常0.8-0.95） （2）节约能量成本 节能的经济效益为： C = B · P C：节约的成本（单位：元） P：燃料单价（单位：元/kg 或 元/m³）
• 假设某工厂废气流量为10,000 m³/h，温度从300℃降至100℃，废气密度为0.946 kg/m³，比热容为1.0 kJ/(kg·℃)，换热器效率为0.8，天然气发热量为35,000 kJ/m³，锅炉效率为0.9，天然气单价为3元/m³。 步骤： 质量流量：m = 10,000 · 0.946 = 9,460 kg/h 理论回收热量：Q = 9,460 · 1.0 · (300 - 100) = 1,892,000 kJ/h 实际回收热量：Q实际 = 0.8 · 1,892,000 = 1,513,600 kJ/h 节约燃料量：B = 1,513,600 / (35,000 · 0.9) = 48.02 m³/h 节约成本：C = 48.02 · 3 = 144.06 元/h 结果：每小时节约天然气48.02 m³，节约成本144.06元。
• 废气成分：含水蒸气或腐蚀性气体的废气需调整比热容和换热器材质。 换热器类型：常见余热回收设备（如热管换热器、板式换热器）效率不同，需根据实际参数选择。 运行工况：废气流量和温度可能随生产变化，需取平均值或动态监测。 单位转换：确保单位一致（如kJ、kg、℃），必要时转换（如1 kcal = 4.184 kJ）。 如需针对具体案例（如污泥烘干废气余热回收）进一步计算或优化，请提供详细参数（如废气流量、温度、成分等）！  

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:

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 · ρ

Among 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).

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

(1) Fuel saving

If waste heat is used to replace fuel (such as natural gas, coal), the amount of fuel saved is:

• B: Amount of fuel saved (unit: kg or m³)
• Q_fuel: The lower calorific value of the fuel (unit: kJ/kg or kJ/m³, such as natural gas about 35,000 kJ/m³)
• η_boiler: Combustion efficiency of boiler or heating equipment (usually 0.8-0.95)

(2) Save energy costs

The economic benefits of energy saving are:

• C: Cost savings (unit: yuan)
• P：Fuel unit price (unit: yuan/kg or yuan/m³)

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:

1. Mass flow rate:m = 10,000 · 0.946 = 9,460 kg/h
2. Theoretical heat recovery:Q = 9,460 · 1.0 · (300 - 100) = 1,892,000 kJ/h
3. Actual heat recovery:Qactual = 0.8 · 1,892,000 = 1,513,600 kJ/h
4. Fuel saving:B = 1,513,600 / (35,000 · 0.9) = 48.02 m³/h
5. 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.

• 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.)!