To ventilate a factory building, 5 kg/s of factory air at a temperature of 27 de

Question

To ventilate a factory building, 5 kg/s of factory air at a temperature of 27 degree C is exhausted, and an identical flow rate of outdoor air at a temperature of - 12 degree C is introduced to take its place. To recover some of the heat of the exhaust air, heat exchangers are placed in the exhaust and ventilation air ducts, as shown in Fig. 5-25, and 2 kg/s of water is pumped between the two heat exchangers. The UA value of both of these counterflow heat exchangers is 6.33 kW/K. What is the temperature of air entering the factory?

Explanation / Answer

So let the temperature of water be T, so the gradient at the exhuast will be

dT = 27 - T

and at the inlet dT'will be , dT = T + 12

Power delivered by the exhaust gas will heat up the incoming gas ,hence we can write this balance equation

5 kg/s * 1 * (300-261 ) = 6,33 (300-T)

T= 270 K ( T is the water temperature)

HEnce the water temperature is 270K

Now when air comes in, the heat exchange equation will give

5 * 1 (T' - 261) = 6.33 ( 270 - T') (T' is the inlet gas temperature)

T = 266 K

Hence the inlet temperature would be - 7 degree celcius.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.