Recover energy without crossing the prohibited air path

Select a recovery mechanism from the actual air-quality and layout constraints, then ask whether its added electrical load outweighs the benefit.

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

Supply and exhaust ducts are 60 ft apart. This original exercise prohibits any transfer path between their air streams; only sensible recovery is required.
The supplied run-around option uses separated closed-fluid coils with no air connection. The proposed heat-pipe option requires adjacent ducts. The proposed wheel has no acceptable carryover verification for this exhaust.
The run-around option avoids 22,000 Btu/h of downstream cooling and adds 2.0 kW total purchased fan/pump input. Extra equipment heat is outside that cooling boundary. Displaced cooling COP is 3.0. Use 3,412 Btu/h per kW.

Apply the supplied layout and air-path constraints first
Proposed device	Supplied constraint	Selection consequence
Wheel	Required carryover evidence absent	Cannot approve from the provided evidence
Heat pipe	This proposed unit requires adjacent ducts	Does not fit the stated separated layout
Run-around coils	Separated fluid circuit; no air connection in the stated model	Evaluate duty and electrical penalty

Study calculator, scratch notepad and whiteboard

Expression Trigonometry in degrees

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

Scratch notepad

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

Which supplied proposal can proceed to the net-energy check under the stated separated-duct and prohibited-air-path constraints?

Choose one responseRun-around coils; the supplied fluid circuit connects separated ducts.The wheel, using its recovery efficiency to prove no carryover.The proposed heat pipe, because every heat pipe can connect remote ducts.The greatest recovery duty; apply the gross thermal ranking first.

ConfidenceWhy did you choose this method? Optional personal reasoning, up to 1,000 characters.

2 · Change the condition

Predict before you inspect the calculation.

Total extra purchased fan/pump power rises to 2.4 kW; the avoided thermal duty and cooling boundary stay fixed.

Avoided cooling = 22,000 Btu/h, cooling COP = 3.0, extra purchased input = 2.4 kW, all extra equipment heat outside the downstream cooling boundary, conversion 3,412 Btu/h per kW.

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: energy recovery, effectiveness

Mark the thermal and electrical boundaries separately. Identify the facts you would need to compare another proposed device.

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

Specified original proposal constraints. Real devices require verification of leakage, carryover, fluid integrity, frost control and applicability. Do not infer every wheel is prohibited or every heat pipe requires adjacent ducts.

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 recovery-device-and-penalty and the step in your comment.

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