Discussion on the energy balance calculation of steam-type lithium bromide absorption chillers
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For steam single-effect/double-effect lithium bromide absorption chiller units, the formula for calculating the cooling capacity is: Qc = (1/3600) × qvc × Cc × ρc × (tc1 – tc2). The formula for calculating the heating capacity is: Qi = (1/3600) × qms × (hs1 – hs2) × (1 – L). The formula for calculating the heat discharged by the cooling water is: Qw = (1/3600) × qvw × Cw × ρw × (tw2 – tw1). What is the energy conservation equation relating P to Qc, Qi, Qw, and P? There are two unknowns in the above formulas: the steam flow rate qms and the cooling water flow rate qvw. How can these two parameters be determined? I would appreciate advice from those who are experts in this field! GB18431 only specifies the constraints between the design value and the nominal value, but does not provide an energy balance equation.
Here are three formulas for selecting the cooling capacity of a chiller:
1. Temperature difference and flow rate method
Q = Cp × r × Vs × ΔT
Q: Heat load (kW)
Cp: Specific heat at constant pressure (kJ/kg·°C); e.g., 4.1868 kJ/kg·°C
r: Specific weight (kg/m³); e.g., 1000 kg/m³
Vs: Water flow rate (m³/h); e.g., 1.5 m³/h
ΔT: Temperature difference between water inlet and outlet (°C); ΔT = T2 – T1 = 10°C
Example:
Q = Cp × r × Vs × ΔT
= 4.1868 × 1000 × 1.5 × 10 / 3600
= 17.445 kW
2. Time and temperature rise method
Q = Cp × r × V × ΔT / H
Q: Heat load (kW)
Cp: Specific heat at constant pressure (kJ/kg·°C); e.g., 4.1868 kJ/kg·°C
r: Specific weight (kg/m³); e.g., 1000 kg/m³
V: Total volume of water (m³); e.g., 0.5 m³
ΔT: Temperature difference between water inlet and outlet (°C); ΔT = T2 – T1 = 51°C
H: Time (hours); e.g., 1 hour
Example:
Q = Cp × r × V × ΔT / H
= 4.1868 × 1000 × 0.5 × 51 / 3600
= 2.908 kW
3. Law of conservation of energy method
Q = Input power – Output power
Q: Heat load (kW)
Input power: Power supplied to the chiller (kW); e.g., 4 kW
Output power: Power delivered by the chiller (kW); e.g., 0.3 kW
Example:
Q = Input power – Output power = 4 – 0.3 = 3.7 kW
Is this the formula you were looking for?
1. Temperature difference and flow rate method
Q = Cp × r × Vs × ΔT
Q: Heat load (kW)
Cp: Specific heat at constant pressure (kJ/kg·°C); e.g., 4.1868 kJ/kg·°C
r: Specific weight (kg/m³); e.g., 1000 kg/m³
Vs: Water flow rate (m³/h); e.g., 1.5 m³/h
ΔT: Temperature difference between water inlet and outlet (°C); ΔT = T2 – T1 = 10°C
Example:
Q = Cp × r × Vs × ΔT
= 4.1868 × 1000 × 1.5 × 10 / 3600
= 17.445 kW
2. Time and temperature rise method
Q = Cp × r × V × ΔT / H
Q: Heat load (kW)
Cp: Specific heat at constant pressure (kJ/kg·°C); e.g., 4.1868 kJ/kg·°C
r: Specific weight (kg/m³); e.g., 1000 kg/m³
V: Total volume of water (m³); e.g., 0.5 m³
ΔT: Temperature difference between water inlet and outlet (°C); ΔT = T2 – T1 = 51°C
H: Time (hours); e.g., 1 hour
Example:
Q = Cp × r × V × ΔT / H
= 4.1868 × 1000 × 0.5 × 51 / 3600
= 2.908 kW
3. Law of conservation of energy method
Q = Input power – Output power
Q: Heat load (kW)
Input power: Power supplied to the chiller (kW); e.g., 4 kW
Output power: Power delivered by the chiller (kW); e.g., 0.3 kW
Example:
Q = Input power – Output power = 4 – 0.3 = 3.7 kW
Is this the formula you were looking for?
No, the examples you gave are just formulas for calculating the cooling capacity of the unit; they represent one of the values, Qc, that I mentioned earlier. What I want to know is what the energy conservation formula is between Qc, Qi, Qw, and P And also, how to determine the steam flow rate qms and the cooling water flow rate qvw when Qc is known
Then I won’t know; I’ll just wait for the expert to reply
It seems that people still lack interest in purely theoretical things! A sample of a domestic lithium bromide chiller was found online, and the details are summarized as follows: 1. At 0.8 MPa and saturated steam, with a chilled water temperature of (12/7°C) and a cooling water temperature of (32/38°C), the energy consumption per unit of cooling capacity for heating is 1.25 kg/(h•kW). 2. At 0.6 MPa and saturated steam, with the same chilled water and cooling water temperatures, the energy consumption per unit of cooling capacity for heating ranges from 1.285 to 1.313 kg/(h•kW), with an average value of 1.298. 3, 0.4 MPa, saturated steam, cold water temperature (12/7°C), cooling water temperature (32/38°C); energy consumption per unit of cooling capacity by the heating source: kg/(h•kW) = 1.371–1.395, with an average value of 1.385. The steam consumption can be roughly estimated through this. But the energy conservation equation among Qc, Qi, Qw, and P is still unknown! I’ll just wait for the experts to give me some advice! Please do not sink the post