Displacement Ventilation For Hospital Patient Room

In displacement ventilation systems, as opposed to the traditional overhead supply mixing ventilation systems, cold air which is supplied at low velocity near the floor captures sensible heat from the space, and rises due to buoyancy, and thus, creates a stratified temperature distribution – cold air near the floor and hot air near the ceiling. This potentially can help in displacing airborne contaminants from the floor to the ceiling, and thus, can create a better indoor environment in the occupied zone. ASHRAE/ASHE Standard 170 – Ventilation of Health Care Facilities has a provision for displacement ventilation for patient rooms.

Recently we performed a comparative study between the traditional overhead supply and displacement ventilation system for a typical patient room using Computational Fluid Dynamics (CFD). Under identical cooling loads the overhead supply system shows recirculating inductive airflow patterns which results in relatively uniform air temperature distribution in the room. Whereas the displacement system shows stratified layers of temperature with predominantly horizontal and vertical airflow patterns. Additionally, thermal comfort analysis indicates displacement system can provide relatively better thermal comfort for occupants at relatively low supply airflow rate. In the case of overhead supply the airborne particles released from the patient’s face can get entrained back into the supply air stream and eventually spread into the entire room. Whereas in the case of displacement system the buoyancy forces keep these particles afloat near the ceiling creating relatively cleaner environment in the occupied zone.

The sensitivity analysis for supply airflow rate or Air Changes per Hour (ACH) indicates that in the case of displacement ventilation 6 ACH and 2 ACH can create respectively too cold and too hot thermal sensation for the occupants whereas the 4 ACH can create thermal neutral environment with optimum conditions for thermal comfort. It should be noted the dynamics of displacement ventilation can change with varying cooling loads and may require modifications in the supply airflow rate and/or supply air temperature to maintain the similar level of thermal comfort. Future study will involve sensitivity study of the displacement ventilation system for various cooling loads in a patient room.
Numerical computations for this study were performed using ANSYS/Fluent CFD software.