A heat exchanger is used to heat cold water at 8 C entering at a rate of 1.2 kg/s by hot air at 90 C entering at rate of 2.5 kg/s. The heat exchanger is not insulated, and is loosing heat at a rate of 28 kJ/s. If the exit temperature of hot air is 20 C, the exit temperature of cold water is: (use constant specific heats at room temperature)

The exit temperature of cold water is 37.6°C

Explanation:

Mass flow rate of water,

Mass flow rate of air,  

Applying the conservation of heat law between air, water and the surrounding:

We get the exit temperature of cold water by making   the subject of the formula above:

when insulating materials rub against each other, they may become electrically charged . electrons , which are negatively charged, may be 'rubbed off' one material and on to the other. the material that gains electrons becomes negatively charged. the material that loses electrons is left with a positive charge.