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Decision Lab · L6 · Actual volume and saturation over ice

Cold air: separate mass basis from frost-point lookup

A familiar sea-level shortcut is not a substitute for the stated cold-air properties. A below-freezing vapor lookup also needs the correct saturation reference.

Original case · Suggested 20 minutes · Edition 2026-10-12.1

By the end of this workshop

  • Choose a method that respects the supplied conditions.
  • Explain how a changed condition affects the decision.
  • Identify a physical or procedural cross-check and an applicability limit.
  • Record the assumption you would verify next time.

Start with the complete model

Givens and assumptions

  • A dry-air heating process carries 1,500 actual ft³/min measured at the inlet. Specific volume at that same local inlet state is supplied as 11.6 ft³/lb dry air; heat capacity is 0.24 Btu/(lb·°F). Use this supplied actual-flow property basis rather than assuming standard air or sea-level density.
  • Dry bulb rises from −14°F to −2°F. No moisture or phase-change duty is included in this specified dry-air model.
  • Separately, use the hypothetical ice-saturation lookup supplied in the changed condition for interpolation practice only. These synthetic values are not accurate water/ice properties or a handbook chart.
Study calculator, scratch notepad and whiteboard

Supports +, −, *, /, ^, parentheses, sqrt, abs, log (base 10), ln, sin, cos, tan, pi and e. Confirm units independently.

Scratch text and drawings stay in this tab and clear when you leave or reload. Save important reasoning in a response or reflection below before leaving.

Scratch whiteboard

Sketch a control volume or air path with a mouse, pen or touch. Ink stays in this page session and clears on reload. Use the response and reflection fields as a keyboard alternative. Sign in before starting and save a reflection to retain your reasoning.

1 · Choose the method

What supplied basis determines sensible duty for this 1,500 actual-cfm dry-air heating process?

Choose one response

2 · Change the condition

Predict before you inspect the calculation.

A separate moist-air state has vapor partial pressure 0.0068 psia. Use a saturation-over-ice table to estimate its frost point.

  • Hypothetical interpolation table, not real ice-property data: the supplied pressure is 0.0050 psia at −20°F and 0.0090 psia at −10°F.
  • For this separate moist-air state, vapor partial pressure is 0.0068 psia. Use linear interpolation only within this supplied interval.

Check the first decision to open the changed-condition response.

Personal notes are not automatically graded. Workshops are linked learning experiments, separate from timed diagnostics and mocks. They do not predict a pass or award instructional hours.

Reference navigation drill

Search concept: psychrometrics, specific volume, saturation over ice

Find the pressure and phase basis before using a property chart. Distinguish a frost point from a liquid-water dew-point lookup.

Use an authorized reference. A search term does not guarantee that a current examination reference contains every relationship used in this teaching case.

Where this model stops

The synthetic lookup values are hypothetical interpolation data, not accurate saturation pressures. Do not use this result for a real frost prediction. A real wet-coil or freezing process needs the actual vapor, condensate/ice and energy balances.

The PE Mechanical: HVAC and Refrigeration examination uses the NCEES PE Mechanical Reference Handbook. These source links provide study context; they are not additional exam-day references.

Found an unclear condition or calculation? Include workshop cold-air-mass-and-frost and the step in your comment.

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