Enthalpy Wheel on Class 4 Laboratory Exhaust

Sensible recovery at design

Q = 1.08 × 24,000 × 0.70 × (72 − (−5)) = 1.08 × 24,000 × 0.70 × 77 ≈ 1,396,000 Btu/h (1.40 MMBtu/h)

Without recovery, the preheat coil would need 1.08 × 24,000 × 77 ≈ 2.00 MMBtu/h; recovery removes about 70% of it and cuts the coil and gas-piping size accordingly.

Exhaust-side leaving temperature

T_ex,leaving = 72 − 0.70 × (72 − (−5)) = 18°F

Annual savings

1.40 MMBtu/h × 1,800 h = 2,520 MMBtu/yr × $9/MMBtu ≈ $22,700/yr

Against a $90,000 premium for the run-around loop, the simple payback on the difference is about four years - real, but far from overwhelming, and it does not account for the fan energy the loop adds or the wheel's purge fan and pressure-drop penalty. Present it as a four-year premium for contamination-free transfer, not as an energy penalty.

The exhaust leaves the wheel at 18°F, and the exhaust carries W ≈ 0.0065 (dew point near 45°F). Wherever the wheel matrix is below 32°F the moisture condenses; below about 20-25°F it freezes. At design, most of the exhaust-side face is well under 20°F, so the wheel will frost within hours at design conditions. The 3% purge does nothing for this - frost control is a temperature problem, not a carryover problem.

Required preheat to hold the exhaust leaving at about 27°F (frost margin)

27 = 72 − 0.70 × (72 − T_pre) → T_pre ≥ 7.7°F

So preheat the outdoor air to roughly 8-10°F, not simply to 0°F. Options, in order of preference:

• Preheat to the calculated setpoint only below the lockout ambient temperature; a wheel spends most of its hours well above design.
• Modulate wheel speed down (reduced effectiveness, higher exhaust leaving temperature) before adding preheat - cheaper than gas in most rate structures.
• Provide a bypass damper for extreme cold and a frost-sensing sequence (exhaust-side ΔP rise or a temperature switch on the exhaust leaving the wheel).
• Note that an enthalpy wheel also transfers moisture, which raises the frost point of the exhaust-side air and makes frosting start earlier than a sensible-only wheel. The 70% sensible-only effectiveness assumption is optimistic at design.

Contamination

Chemistry-hood exhaust is Class 4 (at best Class 3) under ASHRAE 62.1. Class 4 air may not be recirculated or transferred into a Class 1 space, and energy-recovery devices must limit carryover to no more than 10% of the exhaust. A 3% purge plus seal leakage may nominally fall inside that limit on paper, but on a fume-hood system the number that matters is the worst-case carryover of a specific solvent at a specific concentration, and that requires the industrial hygienist's input, not a generic purge percentage. Any wheel also requires the supply side to be held at a higher pressure than the exhaust side, which is hard to guarantee when the lab is under negative pressure by design.

Decision

• The run-around loop is an indirect device: there is zero cross-contamination by construction, and it needs no purge, no pressurization discipline, and no hygienist sign-off on carryover. That is what the $90,000 buys.
• If the wheel is kept, require: a minimum 10% purge rather than 3%, documented supply-side pressurization with a monitoring point, a carryover analysis signed by the industrial hygienist, and explicit AHJ acceptance before bid.
• Weigh the downside: if the AHJ rejects the wheel at submittal, the 24,000 cfm of ductwork, the penthouse layout, and the preheat sizing all get redesigned. On a lab, buy the indirect device and bank the $22,700/yr as a secondary benefit rather than the deciding one.