---
title: "Basic psychrometric processes"
subject: "Applied Thermodynamics"
chapter: 4
key: "semester_3/applied_thermodynamics:basic-psychrometric-processes"
source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/basic-psychrometric-processes
site: BCT Engineering
---

# Basic psychrometric processes

Applied Thermodynamics · Chapter 4: Air-Conditioning

- Asked in: year not given, 6 marks
- Video: [Psychrometric Processes: Heating | Cooling | Humidification | Dehumidification](https://www.youtube.com/watch?v=TRKDhHQc2ys) — SELF ENGINEER
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/basic-psychrometric-processes
- Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-4

## Question

Explain the following psychrometric processes with sketches on the psychrometric chart and give the energy equations: (a) sensible heating, (b) sensible cooling, (c) cooling and dehumidification, (d) heating and humidification.

## Short answer

Sensible heating/cooling holds ω constant, moving horizontally; Q = ṁₐ(h₂−h₁). Cooling and dehumidification occurs below dew point with moisture removal, Q = ṁₐ(h₁−h₂) − ṁ_w h_w, ṁ_w = ṁₐ(ω₁−ω₂). Heating and humidification adds moisture: ṁ_w = ṁₐ(ω₂−ω₁), Q = ṁₐ(h₂−h₁) − ṁ_w h_w.

## Answer

Let $\dot m_a$ be the mass flow of dry air, and 1 and 2 the inlet and outlet states.

#### (a) Sensible heating
Heat is added with no change in $\omega$. The state moves horizontally to the right; DBT, WBT and enthalpy rise, RH falls.
$$
Q = \dot m_a (h_2 - h_1) = \dot m_a\,c_{pm}(t_2 - t_1)
$$
with humid specific heat $c_{pm} = 1.005 + 1.88\,\omega$ kJ/kg K. Done by an electric or steam heating coil.

#### (b) Sensible cooling
Air is cooled by a coil whose surface temperature is above the dew point, so $\omega$ stays constant. The state moves horizontally to the left; RH rises. The same equation applies with the sign reversed.

#### (c) Cooling and dehumidification
The coil surface is below the dew point of the entering air, so vapour condenses. The state moves down and to the left.
$$
Q = \dot m_a (h_1 - h_2) - \dot m_w h_w,\qquad \dot m_w = \dot m_a(\omega_1 - \omega_2)
$$
The load splits into sensible and latent parts, $Q = Q_s + Q_l$. This is the main summer air-conditioning process.

#### (d) Heating and humidification
Used in winter: air is heated, then water or steam is added. Specific humidity and temperature both rise.
$$
\dot m_w = \dot m_a(\omega_2 - \omega_1),\qquad Q = \dot m_a(h_2 - h_1) - \dot m_w h_w
$$

```text
 w ^
   |              o 2d  (d)
   |             /
   |  2b o<-----o 1 ---->o 2a
   |           /
   |      2c o            (c)
   +------------------------> DBT
```
(a) sensible heating, (b) sensible cooling, (c) cooling and dehumidification, (d) heating and humidification.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. During sensible heating on the chart, the state point moves:
   A. Horizontally right, omega constant, RH falls
   B. Vertically up
   C. Down and to the left
   D. Horizontally left, RH rises
   Answer: A. No moisture is added, so omega is constant; DBT rises and RH falls. (Key point: Sensible heating)

2. Why does omega stay constant in sensible cooling?
   A. The coil surface is above the dew point, so no condensation occurs
   B. The coil surface is below the dew point
   C. Water is sprayed into the air
   D. Air velocity is zero
   Answer: A. If the coil surface temperature stays above the dew point of the air, no vapour condenses. (Key point: Sensible cooling condition)

3. In cooling and dehumidification, the condensate rate is given by:
   A. m_w = m_a (omega_2 - omega_1)
   B. m_w = m_a (omega_1 - omega_2)
   C. m_w = m_a (h1 - h2)
   D. m_w = m_a c_pm (t1 - t2)
   Answer: B. Omega drops across the coil, so water removed = m_a (omega_1 - omega_2). The load also splits into sensible and latent parts. (Key point: Cooling and dehumidification)

4. In winter heating and humidification, what happens to the state?
   A. omega rises and temperature rises
   B. omega falls and temperature rises
   C. omega constant, temperature falls
   D. omega rises and temperature falls only
   Answer: A. Air is heated and then water or steam is added, so both DBT and specific humidity increase. (Key point: Heating and humidification)

## Related in Applied Thermodynamics

- [Construction of the psychrometric chart](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Apsychrometric-chart-description) — Applied Thermodynamics, chapter 4
- [Sensible heating of air numerical](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Asensible-heating-load-numerical) — Applied Thermodynamics, chapter 4
- [Cooling and dehumidifying coil load](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Acooling-dehumidifying-coil-numerical) — Applied Thermodynamics, chapter 4

## Related in other subjects

- [Heating and cooling time constants](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aheating-cooling-time-constants) — Electric Machine Design, chapter 2, asked 3×
- [Transformer heating and cooling time constants (25°C/37.5°C)](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atransformer-heating-cooling-constants) — Electric Machine Design, chapter 2, asked 3×
- [Transformer heating and cooling time constants (25°C/40°C)](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atransformer-cooling-constant-45c) — Electric Machine Design, chapter 2, asked 3×
- [Air: PV^3 expansion, cooling, heating; total work](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aair-polytropic-cooling-heating-total-work) — Fundamental of Thermodynamics & Heat Transfer, chapter 2