Troubleshooting the Air Curtain on an Open Display Case

When investigating problems with an open display case, a technician must look not only at the mechanical refrigeration circuit, but also at the air curtain as the possible source of the problem.  When an air curtain becomes disturbed, warm store air can enter the case and cause an escalation of the case temperature and its product.

Many factors can disrupt an air curtain and a service technician needs to identify the source and correct the problem.

Another way an air curtain can be disturbed is by how product is loaded in a case.  Each case is normally marked with a load limit line.  If this line is exceeded, product may be in the path of the air curtain and cause store air to enter the case.  Therefore, product should not be loaded beyond the load limit line.

A service technician can usually identify an air curtain disturbance in one of two ways.  The first is to place a thin plastic bag in front of the case and the air curtain.  (The bags used for supermarket produce work very well.) If the bag is drawn into the case so will the store air, and the cause needs to be identified.

The second way to test the integrity of an air curtain is to smoke the case.  By using a smoke candle to introduce smoke into the return air grille, a service technician can easily see if the air curtain is intact or is being disturbed.  Smoke candles can burn from 30 to 60 seconds and can be found at most local HVAC/R supply houses.  A 30-second burn is usually sufficient to test an air curtain.

Problems with an air curtain can also be associated with the velocity of the air as it falls down in front of the case.  The velocity must be sufficient to keep the air curtain intact.  If the air velocity is not adequate it may cause store air to enter the case.  Problems with the velocity of the air curtain can usually be associated with either an inoperative fan or an iced section of the evaporator coil.

The velocity of the air curtain can be measured at the supply outlet at the top of the case.  It can then be compared to the manufacturer’s guidelines for the case.  It is important to obtain the correct velocity from the case manufacturer, as they may vary from case to case and from manufacturer to manufacturer.

As a general guideline, medium temperature single deck cases will have an approximate discharge velocity of 120 to 140 feet per minute.  Single deck low temperature cases will have a discharge velocity of approximately 200 to 250 fpm.  Multi-deck medium temperature cases will have a discharge velocity of approximately 170 to 400 fpm. (This depends on the application and manufacturer.)

The primary air curtain of a multi-deck low temperature case will have a discharge velocity of approximately 550 fpm; the secondary air curtain will have a velocity of approximately 400 fpm; and the ambient air curtain 250 fpm.

Troubleshooting the air curtain on open display cases can be difficult at times, and a technician should not jump to any early conclusions as to the cause of the problem.  A technician should always measure the discharge air velocity of the air curtain and look for sources of disturbance.  By doing this he should be able to discover most problems associated with these air curtains.