Are fluctuating pressures wrecking your freezer rooms?

NEWS & MEDIA | 08 October 2018

Are fluctuating pressures wrecking your freezer rooms?

For anyone who has seen the small hatches next to cold store doors or in the refrigeration pods and didnt know how important they are, please read the following article from Keith Bell at Cooltec.


When designing freezer rooms, who is responsible for determining how many air release valves are required in the room? Is it the insulated panel suppliers, the refrigeration contractors or is it the consultants. The answer could well be no one.

Traditionally suppliers follow the old Worthington Smith standard, which was one at the freezer door and one in each pod.
Over the years Cooltec has inspected freezer facilities where fluctuating pressure shows its ugly signs i.e.: moving of the corner angles, bowing of the wall panels and kinking of wall and ceiling panels. This results in moisture ingress into the panels, where the silicone seals have been damaged due to the movement. Eventually the panels become waterlogged and frozen.
Some facts about fluctuation pressures inside freezer rooms. Have you ever wondered why your home freezer door sticks after you close it? When opened, warm air enters the freezer, this warm air is of the same volume and pressure as the outside atmosphere, but when you close the freezer door, the air chills down, and decreases in volume, creating a vacuum. This is no different to a large commercial freezer room.
Pressure release valves are designed to balance over pressure or under pressure (vacuum) in the freezer rooms and areas up – 30 ℃. These are fitted with flaps that open when there is a differential pressure and close when the room has balanced.
One can ask what pressure build up in a freezer is dangerous; we say any pressure is dangerous.

The following Graph show differential pressure recorded in Pa based on an air
density 1,2kg/m.

Above: Flow rate - m3 per hour
Above: Flow rate – m3 per hour

 

The optimum pressure in a freezer room should remain constant at 100 PA and air
release valves should regulate the differential pressure within the freezer room.
Any pressure above 100 Pa will start to lift suspended ceilings and bow the wall
panels outwards. Any pressue below 100 Pa will create a vacuum which will pull the suspended ceiling panels and wall panels inwards. This will result in kinks appearing on the outside of the panels. Any fluctuating pressures will cause damage to the silicone on the panel joints.

Causes of over/under pressure in freezer rooms

  • Door standing open for long periods of time, warm air shrinks when cooled causing a negative pressure in the freezer.
  • Incorrect settings on refrigeration unit coolers, especially during the defrost cycle where excessive heat is used.
  • Insufficient air release valves installed.
  • Incorrect positioning of air release valves.
  • Rapid cooling within the room.

Recommended position for air release valves:

 

Recommended position for air release valves

 

  • Remedial actions that can be taken to prevent damages caused by over / under pressure in freezer rooms.
  • Motorise cold room doors.
  • Check the defost settings on refrigeration equipment.
  • Check the quantity of air release valves with the operational requirements of the rooms.
  • Check the position of the air release valves, these should not draw warm air into the room.
  • Check all air release valves daily for blockages and / or damage.
  • The best locations for air release valves are at high level just below the ceiling panels and at each door opening.

Example of a blocked air release valve

Example of a blocked air release valve

 

If you have any questions call Keith directly on 076 441 5970 or email cooltec@webafrica.org.za for a full survey of your freezer