Reheat Versus Face-and-Bypass in a Hospital Operation Theatre AHU
The 40-bed district hospital is commissioning a two-theatre operation suite. Design room condition is 22°C DBT and 50% RH; outdoor design is 42°C DBT and 27°C WBT. The calculated room loads are 48 kW sensible and 12 kW latent. The AHU supplier proposes a chilled-water coil with apparatus dew point (ADP) 9°C and bypass factor (BF) 0.12, followed by a reheat coil. The supply-air quantity is fixed at 6.0 kg/s by duct size. The supplier argues that reheat is wasteful and offers to increase the BF to about 0.40 by opening the face-and-bypass damper, claiming this will deliver air at room-neutral temperature and remove the reheat coil from the drawing. The hospital engineer asks you to review the proposal before the equipment is ordered. Take cp of moist air as 1.02 kJ/kg·K and latent heat of vapour as 2450 kJ/kg. Room humidity ratio at 22°C/50% RH is approximately 0.0082 kg/kg dry air; saturated humidity ratio at 9°C ADP is approximately 0.0071 kg/kg dry air.
1. Determine the off-coil temperature and reheat load with the supplier's original BF = 0.12, and state whether the coil alone can meet the room sensible load.
Read the worked answer
Required supply temperature: 48 = 6.0 × 1.02 × (22 − T_supply), so T_supply = 22 − 48/6.12 = 14.16°C.
With BF = 0.12, off-coil temperature = ADP + BF × (room DBT − ADP) = 9 + 0.12 × (22 − 9) = 10.56°C.
Reheat load = 6.0 × 1.02 × (14.16 − 10.56) = 22.0 kW.
The coil alone leaves the air at 10.56°C, which is colder than the 14.16°C required to absorb 48 kW sensible load at the fixed air quantity. Therefore the coil alone cannot meet the sensible load without either reducing airflow, raising the ADP, or adding reheat. Since airflow is fixed by duct size and ADP is selected for latent control, reheat is functionally necessary in this design.
2. Evaluate the supplier's BF = 0.40 proposal: compute the off-coil condition and latent removal, and decide whether the reheat coil can be deleted.
Read the worked answer
Off-coil temperature at BF = 0.40: 9 + 0.40 × (22 − 9) = 14.2°C. This is almost exactly the required supply temperature, so sensible reheat would be nearly zero.
Off-coil humidity ratio: W_off = W_ADP + BF × (W_room − W_ADP) = 0.0071 + 0.40 × (0.0082 − 0.0071) = 0.00754 kg/kg.
Moisture removed = 6.0 × (0.0082 − 0.00754) = 0.00396 kg/s.
Latent removal = 0.00396 × 2450 = 9.7 kW, against a room latent load of 12 kW.
The coil would under-remove about 2.3 kW of latent heat. Room humidity would drift upward, which is unacceptable for an operation theatre. The reheat coil cannot be deleted merely to save energy; doing so trades sensible-energy savings for loss of humidity control. A separate dehumidification step or a lower ADP with reheat would be needed if the reheat coil is to be avoided.
3. Recommend a practical design and control strategy, and explain the operating penalty of the original scheme compared with your recommendation.
Read the worked answer
Retain latent control first: keep the coil capable of ADP around 9°C, or go lower if the summer latent load is uncertain, and use reheat only as needed. A better arrangement is to reset ADP upward during part-load and low-latent periods, and to use modulating chilled water with a room RH controller rather than a fixed reheat duty. Reheat should come from waste heat, such as chiller condenser heat or a heat-recovery coil, not electric resistance.
The original 22 kW reheat load is the penalty for decoupling sensible and latent control at full load. If electric reheat ran for 6,000 hours, it would consume about 132,000 kWh per year, which is a large operating cost. The supplier's zero-reheat proposal avoids that cost but fails the latent load by about 2.3 kW and will cause high OT humidity. The correct judgement is not 'reheat is always wasteful' or 'bypass is always efficient'; it is that humidity control in an OT is non-negotiable, and reheat should be minimised through ADP reset, heat recovery, and part-load control rather than eliminated by opening the bypass damper.