Pressurization in a controlled food process area is critical to counteract the infiltration of air through room openings.
Infiltration can be caused by wind, temperature differences and negative pressures from exhaust or process equipment. Infiltration can cause temperature fluctuations in a controlled space, employee discomfort, carry airborne contamination and increase the likelihood of condensation within the area.
When wind impinges on a building, it creates a pressure on the building exterior. This pressure depends upon wind direction and building location. Wind pressures are generally positive on the windward side of the building. These positive pressures, up to 0.5-inches water gauge, can cause very high velocities of air through openings in the building exterior. It is essential that doors be tight-fitting and remain closed as much as possible.
As wind flows around and over a building top, a negative pressure is created on the downwind side of the building. These forces can actually draw air out of a building. Therefore, a building has two forces acting on it because of wind: positive and negative.
In order to counteract wind forces on a building, maintain and keep doors in good working condition and keep them closed. Infiltration can come through cracks and crevices around the doors-even with the doors closed. A positive pressure in the building will help counteract these remaining wind forces. In critical process areas, it is a good idea to have no openings or doors that go directly to the outside.
Temperature differences between the outside of the building and a controlled interior room, or between two adjoining interior rooms, can also cause air transfer. Because of air density differences, warm air from the higher-temperature room will tend to rise out of the top of a door or other wall openings. Corresponding "cooler" air from the lower temperature room will then be replaced at the bottom of the door. This is very evident in low-temperature rooms, such as freezers.
In many cases, the warm air comes from outside and causes severe frost and ice problems around the door opening. The cold air then falls out of the bottom of the door causing drafts and possibly condensation on the floor outside the refrigerated room. In higher-temperature rooms, the same forces are in effect but are not as obvious as those in a freezer application.
Counteract the airflow
A door is the best way to counteract the airflow through an opening due to temperature and density differences. If openings and doors are necessary in higher-temperature applications, an air curtain over the door will help break up and minimize the transfer of air between the two rooms. Positive pressurization will greatly improve the efficiency of the air curtain in controlling the air transfer between the rooms.
In lower-temperature applications, an air curtain/vestibule combination is a good critical control solution. The air curtain will help control the infiltration into the vestibule. A fan coil cooling unit in the vestibule will help remove much of the moisture and heat from the remaining infiltration before it enters the cold room. Depending upon the application, a reheat coil may also be necessary. This system will substantially reduce the possibility of frost and ice accumulation around the cold room.