Basic HVAC Processes

  • Explain the basic principles of heating, ventilation, air conditioning, and refrigeration.

  • Sensible Heating or Cooling:

Sensible Heating or Cooling

A psychrometric process that involves the increase or decrease in the temperature of air without changing its humidity ratio.

\[\dot{Q} = \dot{m_a}(h_2-h_1)\]

  • Heating and Humidifying:

Heating and Humidifying

A psychrometric process that involves the simultaneous increase in both the dry bulb temperature and humidity ratio of the air.

  • Adiabatic or Evaporative Cooling:

Adiabatic or Evaporative Cooling

A psychrometric process that involves the cooling of air without heat loss or gain. Sensible heat lost by the air is converted to latent heat in the added water vapor.

Here, the wet bulb temperature, \(T_{WB}\) remains constant. \[h_2 = h_1 + h_3(\omega_2 - \omega_1)\] \[\dot{m_w} = \dot{m_a}(\omega_2 - \omega_1)\] \[h_3 = h_f \ at \ T_{WB}\]

  • Adiabatic Mixing of Moist Air Stream:

Adiabatic Mixing of Moist Air Stream

A psychrometric process that involves no net heat loss or gain during the mixing of two air streams.

\[\dot{m_{a3}} = \dot{m_{a1}} + \dot{m_{a2}}\] \[h_3 = \dfrac{\dot{m_{a1}}h_1 + \dot{m_{a2}}h_2}{\dot{m_{a3}}}\] \[\omega_3 = \dfrac{\dot{m_{a1}}\omega_1 + \dot{m_{a2}}\omega_2}{\dot{m_{a3}}}\] distance \(\bar{13}\) = \(\dfrac{\dot{m_{a2}}}{\dot{m_{a3}}}\) \(\times\) distance \(\bar{12}\) measured on the psychrometric chart

Solved Example:

78-1-01

The by-pass factor for a cooling coil:

Correct Answer: B

Solved Example:

78-1-02

During sensible cooling,

Correct Answer: D

Solved Example:

78-1-03

When the air is passed through an insulated chamber having sprays of water maintained at a temperature higher than the dew point temperature of entering air but lower than its dry bulb temperature, then the air is said to be:

Correct Answer: A

Solved Example:

78-1-04

In a pressure enthalpy chart, the space to the left of the saturated liquid line represents:

Correct Answer: C

Solved Example:

78-1-05

During sensible heating of air ______________ decreases.

Correct Answer: B

Solved Example:

78-1-06

In order to cool and dehumidify a stream of moist air, it must be passed over the coil at a temperature:

Correct Answer: C