11.2 The Eight Air-Conditioning Processes and the Process Triangle

11.2 The Eight Air-Conditioning Processes and the Process Triangle

The HVAC Excellence task list requires the technician to "describe the eight processes of air conditioning and how to plot each on a psychrometric chart," and to "define and use the Process Triangle on the psychrometric chart to calculate sensible heat, latent heat and total heat." These are two halves of one skill: identify what the equipment is doing to the air, then quantify it.

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1. The Eight Processes

Every change to an airstream moves its state point in one of eight directions from the origin point. Four are pure; four are combinations.

                       Humidifying
                            ^
                            |
        Cooling &           |          Heating &
        Humidifying    \    |    /     Humidifying
                        \   |   /
                         \  |  /
    Sensible <------------ ORIGIN ------------> Sensible
    Cooling              /  |  \                Heating
                        /   |   \
        Cooling &      /    |    \    Heating &
        Dehumidifying       |         Dehumidifying
                            |
                            v
                      Dehumidifying

Two of these deserve special attention.

Process 5 — the cooling coil. Air entering a coil whose surface is below the entering air's dew point is cooled and dehumidified simultaneously. The process line runs from the entering-air state point in a straight line toward the coil's apparatus dew point on the saturation curve. How far along that line the air actually travels depends on bypass factor — the fraction of air that passes between fins without contacting a cold surface. Slow air over a deep, clean coil has a low bypass factor and travels far down the line (cold, dry supply air). Fast air over a shallow or fouled coil has a high bypass factor and stops short (warmer, wetter supply air, poor dehumidification).

Process 7 — evaporative cooling. Water sprayed into or evaporated from a media pad absorbs its latent heat of vaporization from the airstream itself. Sensible heat leaves the air and returns as latent heat in the added vapor, so total heat is essentially unchanged. The process is therefore adiabatic and follows a constant wet-bulb (constant enthalpy) line down and to the right. This is why evaporative cooling works in Phoenix (large wet-bulb depression, lots of room to move) and fails in Houston (small depression). The theoretical limit is the entering air's wet-bulb temperature; practical media reach 70–90% of that depression, called the saturation effectiveness.

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2. The Process Triangle

A cooling coil's process line is a diagonal. The process triangle decomposes it into two right-angle legs whose meanings are unambiguous:

   Return air point (1)  ●────────────────────┐  ← horizontal leg = SENSIBLE
                          \                   │
                           \  (hypotenuse     │  vertical leg = LATENT
                            \   = TOTAL)      │
        Supply air point (2) ●────────────────┘

• The horizontal leg connects the two points at constant humidity ratio: it is the sensible change (temperature only).
• The vertical leg connects at constant dry bulb: it is the latent change (moisture only).
• The hypotenuse — the actual process line — is the total change.

Because enthalpy accounts for both, the three legs relate as: $Q_{\text{total}} = Q_{\text{sensible}} + Q_{\text{latent}}$

The three airside formulas

$Q_{\text{sensible}} = 1.08 \times \text{CFM} \times \Delta T_{\text{DB}}$ $Q_{\text{latent}} = 0.68 \times \text{CFM} \times \Delta W_{\text{grains}}$ $Q_{\text{total}} = 4.5 \times \text{CFM} \times \Delta h$

Where the constants come from — the task list asks you to "learn the properties of air used to calculate the sensible heat factor," and reconstructing them beats memorizing them:

• $1.08 = 0.075\ \text{lb/ft}^3 \times 0.24\ \text{BTU/lb·}^\circ\text{F} \times 60\ \text{min/hr}$
• $4.5 = 0.075\ \text{lb/ft}^3 \times 60\ \text{min/hr}$
• $0.68 = 4.5 \times 1{,}060\ \text{BTU/lb} \div 7{,}000\ \text{grains/lb}$ (1,060 BTU/lb is the latent heat of vaporization of water at typical coil conditions)

All three assume standard air. At altitude, correct for density — at 5,000 feet, multiply by roughly 0.83.

Worked example

A residential system moves 1,200 CFM. Return air is 75°F DB / 63°F WB (65 grains/lb, 28.6 BTU/lb). Supply air is 55°F DB / 54°F WB (60 grains/lb, 22.6 BTU/lb).

$Q_s = 1.08 \times 1{,}200 \times (75 - 55) = 1.08 \times 1{,}200 \times 20 = 25{,}920\text{ BTU/hr}$ $Q_l = 0.68 \times 1{,}200 \times (65 - 60) = 0.68 \times 1{,}200 \times 5 = 4{,}080\text{ BTU/hr}$ $Q_t = 4.5 \times 1{,}200 \times (28.6 - 22.6) = 4.5 \times 1{,}200 \times 6.0 = 32{,}400\text{ BTU/hr}$

Check: $25{,}920 + 4{,}080 = 30{,}000$, against a total of 32,400 — about 7% apart, which is normal for chart-read enthalpy values and rounding. On the exam the arithmetic is expected to close; in the field, treat a large disagreement as a sign that one of the four measurements is bad.

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3. Sensible Heat Ratio

$\text{SHR} = \frac{Q_{\text{sensible}}}{Q_{\text{total}}}$

Using the numbers above: $\text{SHR} = 25{,}920 \div 32{,}400 = 0.80$.

Why SHR matters practically. Equipment is rated at a design SHR. If the building's actual SHR is much lower than the equipment's, the coil cannot remove enough moisture and the space stays humid even while the thermostat is satisfied. Remedies: lower the airflow per ton (from 400 CFM/ton toward 350 CFM/ton, which lowers coil surface temperature and increases moisture removal), select equipment with a lower rated SHR, add a variable-capacity system that runs longer at part load, or add dedicated dehumidification.

Conversely, running too much airflow raises coil temperature above the dew point, and the coil stops dehumidifying entirely — a common cause of "the house is 72°F and clammy."

On the chart, SHR is a slope. Most charts carry a protractor in the upper-left corner: draw a line from the reference point through the SHR value on the protractor scale, then draw a parallel line through the room design point. That line is the room sensible heat ratio line, and the supply air must be plotted somewhere along it to satisfy the room's combined sensible and latent load.