Refrigeration systems utilizing an outdoor air-cooled condenser located in an area where the outdoor temperature can drop below approximately 70°F must be designed to prevent the system’s discharge pressure from dropping too low. There are several different industry methods that can be used to successfully accomplish this task. The condenser fan motor(s) can be cycled on and off or its speed varied in response to a change in the system’s high side pressure. Another method (though not used very often) is to reduce the airflow across a condenser with some type of damper arrangement.

A popular method used by many designers is to flood the condenser with refrigerant during low ambient temperatures. This approach reduces the effective area of the condenser which in turn causes the discharge pressure to rise. A head pressure control valve installed on the outlet of the condenser restricts the refrigerant flow as the outdoor temperature drops. The refrigerant will begin to back up in the condenser causing it to become flooded with refrigerant.

In order for this valve to properly maintain a minimum high-side pressure, additional refrigerant needs to be added to the system to allow the condenser to become flooded. The system actually requires more refrigerant during low ambient temperatures than during peak load conditions at high outdoor ambient temperatures. The amount of additional refrigerant which must be added to the system is basically a factor of the size of the condenser and the lowest anticipated operating ambient temperature. To determine the additional amount of refrigerant that needs to be added, always refer to the system manufacturer’s guidelines.

TABLE IT

If these guidelines are not available, the head pressure control valve manufacturer may also have a recommended procedure. One suggested method is to first properly charge the system using normal industry methods. Then, weigh in the amount of additional refrigerant required to adequately flood the condenser. To determine the amount of additional refrigerant required, you must first determine the amount of refrigerant required to completely flood the condenser. This is done by using a table supplied by the valve manufacturer. To use this table you will need to record the following information:

• The diameter of the condenser tubing;

• Its total length (including the return bends of the condenser);

• Type of refrigerant used; and

• The lowest ambient conditions to be encountered.

For example, suppose a system utilizing an air-cooled condenser with a total length of 128 feet of 3/8-inch tubing needs to operate properly at an outdoor temperature of 0°F. If the system is using HFC-134a, then using the valve manufacturer’s table you determine the density factor per foot of tubing to be 0.054. This means for each foot of condenser tubing, you need to add 0.054 pounds of additional refrigerant. In this example, you have 128 feet of equivalent tubing, so 6.91 pounds of additional refrigerant needs to be added to completely flood the condenser (128 feet × 0.054 pounds/foot = 6.91 pounds).

However, completely flooding a condenser to achieve proper head control is not necessary. Another table is then used to adjust this value to determine the actual amount of additional refrigerant required to adequately maintain the system’s high-side pressure during the lowest anticipated outdoor temperature.

For example, if our system is operating at a 20° evaporating temperature, using another table you will find the original value of 6.91 pounds needs to be reduced by a factor of 78 percent, so you only really need to add 5.39 pounds (6.91 lbs × 0.78 = 5.39 lbs) of refrigerant to adequately flood the condenser at an outdoor temperature of 0°.

FACTORING IT ALL IN

There is one other concern using this method: You must also factor the current outdoor temperature when originally charging the system. If the outdoor air temperature is already below approximately 70°, the condenser will be partially flooded when charged using standard industry procedures. The weigh-in charge must again be adjusted to compensate for the already partially flooded condenser. To do this, you take the difference between the factor used for the lowest anticipated outdoor temperature and the current outdoor temperature.

For example, suppose the current outdoor temperature is 40°, and at this temperature the chart shows you to reduce the completely flooded charge by a factor of 60 percent.  You must use the difference between the factor of 78 percent (at 0°) and 60 percent (at 40°), so you now use a multiplication factor of only 18 percent (78 percent – 60 percent = 18 percent). That means you now multiply the original 6.91 pounds (completely flooded charge) by 18 percent, which means you now only add 1.24 pounds of additional refrigerant to the system.

Publication date: 11/01/2010