3.2 Psychrometric Processes, Air Mixing, and Coil Diagnostics

Psychrometric Processes, Air Mixing, and Coil Diagnostics

Recognize the direction before calculating

Once a state point is plotted, an HVAC process becomes a path:

The direction is a valuable error check. If an active cooling coil is reported to lower dry bulb while substantially increasing humidity ratio, investigate sensor locations, liquid carryover, humidifier operation, infiltration, or unstable operating mode.

Mixed air and outdoor-air fraction

For two fully mixed airstreams with comparable density, a dry-bulb approximation is:

$T_{ma}=f_{oa}T_{oa}+(1-f_{oa})T_{ra}$

Rearranging:

$f_{oa}=(T_{ra}-T_{ma})/(T_{ra}-T_{oa})$

Suppose outdoor air is 40°F, return air is 75°F, and mixed air is 68°F. The estimated outdoor-air fraction is $(75-68)/(75-40)=7/35=0.20$, or 20%. The result assumes representative temperatures, complete mixing, stable flow, and no material heat gain between the measurement planes.

When the outdoor and return temperatures are close, a small sensor error creates a large percentage error because the denominator is small. Temperature stratification in a mixing box is another common failure. Use a traverse or multiple-point average, compare enthalpy or another method when appropriate, and do not label a single convenient spot reading “mixed air” unless the air is actually mixed.

On the chart, the mixed state lies on the straight line joining outdoor and return states. The distance ratio along that line corresponds to the dry-air mass fractions. If the plotted mixed point falls far off the line, at least one measurement, sampling plane, or process assumption is wrong.

Coil processes and heat

For standard air, sensible heat is commonly calculated as:

Qs = 1.08 × CFM × ΔT

Total air heat uses enthalpy:

Qt = 4.5 × CFM × Δh

The constants are standard-air approximations. Use the nonstandard-air form or project method when density materially differs. Cooling-coil total load should use entering-minus-leaving enthalpy so the result is positive for heat removed. Heating-coil sensible load uses leaving-minus-entering dry bulb.

Example: A coil handles 8,000 CFM, entering enthalpy is 31.0 Btu/lb dry air, and leaving enthalpy is 24.5. Total cooling is 4.5 × 8,000 × (31.0 − 24.5) = 234,000 Btu/h. If airflow or state measurements are uncertain, the calculated load is uncertain too.

Apparatus dew point and bypass behavior

The cooling-and-dehumidification line can be extended toward the saturation curve. Its intersection is an approximate apparatus dew point representing an effective coil-surface condition. Leaving air usually does not reach that saturated point because some air effectively bypasses the coldest surface or has limited contact.

A coil that produces little temperature or moisture change may have low water flow, warm entering water, fouling, air binding, bypass leakage, too much airflow, or inactive controls. Psychrometrics indicates the energy result; it does not by itself identify which cause is responsible.

Diagnostic cross-checks

Use an ordered process:

1. Confirm system mode and control commands.
2. Verify airflow by an appropriate method.
3. Take representative entering and leaving states after stabilization.
4. Plot both points and inspect process direction.
5. Calculate sensible and total heat with consistent signs and units.
6. Compare with water-side heat where water flow and temperature data are reliable.
7. Investigate the largest measurement uncertainty before declaring a coil defect.

Air-side and water-side results will rarely match exactly because of casing heat gain, condensate, leakage, instrument accuracy, and time lag. A large mismatch is a prompt to investigate, not permission to average the two results.

Frequent exam traps

• Using relative humidity as though it were humidity ratio.
• Reversing the outdoor-air fraction numerator.
• Applying the temperature method with nearly equal outdoor and return temperatures.
• Using dry-bulb difference for a cooling coil's total load.
• Mixing CFM measured at one condition with standard-air constants without considering density.
• Sampling mixed air before the outdoor and return streams have blended.

Name the process, draw its expected direction, and then calculate. That order catches more errors than calculator speed.