Psychrometrics on the PE Mechanical exam

Psychrometrics problems on the HVAC & Refrigeration exam are rarely about reading a property for its own sake. They name a process (mix, heat, cool and dehumidify, humidify, evaporatively cool) and ask for a temperature, a moisture rate or a load. The Mechanical Reference Handbook gives you the moist-air property relations and three ASHRAE charts (sea level, high temperature and 5,000 ft) in Chapter 7, and the standard-air load relations, the elevation corrections and adiabatic mixing in Chapter 9. HVAC Loads and Psychrometrics is 18–27 of the 80 questions on the HVAC & Refrigeration exam, and the same skills reach into Thermal & Fluid Systems.

What you actually need to own

• The state. Dry-bulb, wet-bulb, dew point, humidity ratio W, relative humidity, enthalpy h and specific volume. Any two independent properties fix the state. The relation h = 0.240t + W(1,061 + 0.444t) Btu/lb of dry air lets you compute what the chart would give you. W is often quoted in grains: 7,000 grains = 1 lb.

• Each process as a direction on the chart. Sensible heating or cooling runs horizontally, at constant W. Evaporative cooling runs up and to the left along a constant wet-bulb line. Cooling with dehumidification runs down and to the left, losing both dry-bulb and W. Steam humidification raises W at nearly constant dry-bulb; an adiabatic spray raises W along the wet-bulb line, like an evaporative cooler.

• The standard-air factors, each paired with its own driving difference. At sea level:

  • sensible, q = 1.10 × cfm × Δt;
  • total, q = 4.5 × cfm × Δh;
  • moisture, ṁ = 4.5 × cfm × ΔW;
  • latent, q = 4,840 × cfm × ΔW, with W in lb/lb (convert grains first).

  Latent = total − sensible is the cross-check.

• Mixing. Adiabatic mixing is a mass-weighted average of h and W, and the mixed point lies on the straight line between the two states. It sits closer to the larger stream: the segment lengths are inversely proportional to the mass flows. For dry-bulb, the airflow-weighted average of the two temperatures is the plenum shortcut.

• Dew point. A cold surface chills the air next to it at constant W until it saturates. The dew point is where the constant-W line meets the saturation curve.

• Direct evaporative coolers. Leaving dry-bulb = t_db − ε(t_db − t_wb). The wet-bulb depression sets the ceiling, which is why the cooler does more in dry air.

• Altitude. The 1.10, 4.5 and 4,840 factors assume sea level. At elevation, scale each one by the pressure ratio P/P₀. The Handbook gives 0.92, 3.74 and 4,027 at 5,000 ft. At 5,000 ft, read states from the Handbook's 5,000-ft chart, not the sea-level one.

Where people lose points

• A factor paired with the wrong difference. In one coil problem (6,500 cfm, Δh = 8.6 Btu/lb), the right total is 21.0 tons. Using 1.10 with Δh gives 5.1 tons; using 4.5 with the 25°F dry-bulb drop gives 60.9.
• Sensible reported as total. The same coil's sensible load is about 14.9 tons. A coil that lowers the dew point is also removing moisture, so total must exceed sensible.
• Sizing airflow on the total load. Supply airflow comes from the sensible load and the supply-to-room ΔT: 180,000 Btu/h at a 17°F ΔT needs about 9,600 cfm. The latent load is met by the coil at that airflow, not by more air.
• The altitude correction applied the wrong way. Thinner air carries less heat per cfm, so it needs more airflow. In a 5,000-ft problem the corrected answer is about 9,100 cfm. The sea-level answer is 7,600, and multiplying by the pressure ratio instead of dividing gives 6,300.
• Mixing fractions taken from the wrong total. 3,000 cfm outdoor at 100°F with 9,000 cfm return at 76°F mixes to 82°F. Using 3,000/9,000 as the outdoor fraction gives 84°F, and swapping the weights gives 94°F. When outdoor air is below zero, count the full span: from −5°F to 72°F is 77°F, not 72.
• The wrong line to saturation. The dew point follows constant W. Following the wet-bulb line returns the wet-bulb instead: 68°F for 78°F, 60% air whose dew point is 63°F.
• Ignoring the order of components. When a spray sits between the mixing box and the heating coil, the coil sees the air leaving the spray, not the mixed air. Read the arrangement before you pick the entering state.

How to study it

Start from one state point and run every process on it: heat it, cool it below its dew point, mix it with outdoor air, evaporatively cool it. Predict each end state before you compute it. Then work coil problems three ways (sensible, total, latent) and check that latent ≈ total − sensible. Finish with timed problems that hand you only the chart and the Handbook. Reading the chart quickly on screen is its own skill, covered in the psychrometric chart on the CBT, and the psychrometric calculator is useful for checking your reads. The loads these processes serve are in cooling and heating load calculations.

Part of the HVAC Loads & Psychrometrics area of the HVAC & Refrigeration exam (also relevant to Thermal & Fluid Systems). → Start with a free chapter — your exam's first chapter, free with an account.