Buildings with high ventilation rates can significantly reduce their heating oad by recovering energy from the exhaust air stream. One way that this can be done is with a runaround loop, shown in this Figure. A runaround loop consists of two conventional liquid to air cross-flow heat exchangers. An ethylene glycol solution with 35% mass percent glycol is pumped at a rate mg = 1 kg/s through botl heat exchangers. The specific heat of this glycol solution is cp,g = 3.58 kJ/kg-K. The glycol solution is heated by the warm air exiting in the exhaust duct during %3D winter oneration The warm olycol solution is then used to preheat cold gir entering
Buildings with high ventilation rates can significantly reduce their heating oad by recovering energy from the exhaust air stream. One way that this can be done is with a runaround loop, shown in this Figure. A runaround loop consists of two conventional liquid to air cross-flow heat exchangers. An ethylene glycol solution with 35% mass percent glycol is pumped at a rate mg = 1 kg/s through botl heat exchangers. The specific heat of this glycol solution is cp,g = 3.58 kJ/kg-K. The glycol solution is heated by the warm air exiting in the exhaust duct during %3D winter oneration The warm olycol solution is then used to preheat cold gir entering
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Transcribed Image Text:Buildings with high ventilation rates can significantly reduce their heating
load by recovering energy from the exhaust air stream. One way that this can be
done is with a runaround loop, shown in this Figure. A runaround loop consists of
two conventional liquid to air cross-flow heat exchangers. An ethylene glycol
solution with 35% mass percent glycol is pumped at a rate mg = 1 kg/s through both
heat exchangers. The specific heat of this glycol solution is cp,g = 3.58 kJ/kg-K.
The glycol solution is heated by the warm air exiting in the exhaust duct during
winter operation. The warm glycol solution is then used to preheat cold air entering
from outdoors through the ventilation duct.
ventilation duct
exhaust duct
m, = 5 kg/s, T, = 25°C
(3)
me =1 kg/s
(5)
pump
(6)
m, = 5 kg/s, T¡ =-10°C
Outdoor air is blown into the building at a ma = 5 kg/s. The outdoor
temperature is T1 = -10°C. The building is tightly constructed so the exhaust
airflow rate may be assumed to be equal to the ventilation airflow rate (ma = 5
kg/s). The air leaving the building through the exhaust duct is at T3 = 25°C. The
crossflow heat exchangers in the exhaust and ventilation streams are identical,
each having a finned coil configuration and an estimated conductance UA = 10
kW/K.
a) Determine the effectiveness of the ventilation and exhaust heat exchangers.
b) Determine the temperatures of the glycol solution at states (5) and (6).
c) Determine the overall effectiveness of the runaround loop.
d) It has been suggested that the performance of the runaround loop can be
improved by optimizing the glycol flow rate. Plot the run-around loop overall
effectiveness as a function of the glycol solution flow rate for 0.1 kg/s < g « 4 kg/s.
Assume that the conductance of the heat exchangers varies with glycol solution
flow rate to the 0.4 power based on a value of UA = 10 kW/K at mg = 1 kg/s. What
flow rate do you recommend?
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