Climate Control

HVACR Tech Tip: How to Determine Total Refrigerant System Charge When Using Head Pressure Control

Parker Sporlan Head Pressure Control Charging RecommendationsThe design and operation of Parker Sporlan Head Pressure Control Valves are discussed thoroughly in Bulletin 90-30. Installation and service is covered in Bulletin 90-31. This Climate Control blog will provide complete charging instructions, from determining the correct amount of refrigerant to actually charging the system.

If the manufacturer of your equipment provides charging information it should be used. However, if it is not provided, the following procedure is suggested.


Determining the amount of charge

When “refrigerant side” head pressure control is utilized on a system, one of the most important factors is determining the total system refrigerant charge. While on most packaged units the amount of charge is listed on the unit, the required charge for a field built-up system cannot be listed by the manufacturer. The charge is usually added when the system is started up until “proper” system performance is reached. However, this is not satisfactory and if the system is to function properly year-round, the correct amount of extra charge must be calculated ahead of time.


Retrofit precautions

When changing refrigerants on a retrofit, be sure to calculate the refrigerant charge required for the new refrigerant. The density of the alternate refrigerants varies considerably from their CFC predecessors. In other words, if you remove 10 pounds of R-12 from a system being retrofitted to R-401A, do not charge the system with 10 pounds of R-401A. Typically, an R-401A system would only need approximately 90% of the R-12 previously required.

There are two methods of calculating the extra charge necessary to flood the condenser if the condenser manufacturer does not publish this data.

1. Completely Flooded Condenser

The easiest method is to calculate the volume of the condenser coil and then use the density factor of the refrigerant shown in Table 1 on Page 4 of Bulletin 90-30-1 to figure the pounds of refrigerant necessary to completely flood the condenser coil at the appropriate ambient. The factors involved in calculating the extra pounds of refrigerant are:

a. Length of tubing and return bends in condenser

b. The minimum ambient temperature at which systems will be required to function

c. Tubing size and wall thickness

d. Refrigerant

Parker Sporlan LAC-4The primary point to remember in selecting the proper density factor is that when the liquid drain valve (ORI, OROA, or LAC) is throttling, the refrigerant temperature will be at the same temperature as the ambient.

Example: Calculate the extra refrigerant charge necessary for a Refrigerant 22, roof-top, air conditioning unit (40°F evaporator and a minimum condensing temperature of 90°F) with compressor unloading to 33-1/3% of full compressor capacity. To determine the equivalent length of tubing in a condenser proceed as follows: First, count the number of tubes and multiply this by their length.

Example: 150 tubes x 7.55 feet = 1132.5 feet

Next, count the return bends and multiply them by the factor shown in Table 1.

Example: 150 bends x .250 for 1/2 inch bends = 37.5 feet

Then add this 37.5 feet to the 1132.5 feet for a total of 1170 feet.

The system uses a 30 hp condensing unit with a condenser coil containing 1170 equivalent feet of 1/2 inch tubing (tubes and return bend). Assume a design temperature of minus 20°F minimum ambient. From Table 1 we find the density factor necessary to calculate the pounds of extra refrigerant to completely flood the condenser at minus 20°F: 1170 feet x .102 pounds/foot = 119 pounds.

2. Partially Flooded Condenser

On many systems, it isn’t necessary to completely flood the condenser to maintain sufficient operating head pressure (equivalent to approximately 90°F condensing temperature) because of a milder climate than Method 1 assumes. Therefore, a second method is available. The additional information found in Tables 2 and 3 on Page 8 of Bulletin 90-30-1 can be used to figure more closely the charge necessary to properly flood the condenser for sufficient head pressure at various minimum ambient temperatures. (The multipliers are applied to the extra refrigerant charge that was calculated in Method 1 to completely flood the condenser.)

Example: Our example calls for a compressor equipped with unloaders. Since the compressor would unload at the low ambients this must be taken into consideration. This is necessary since as the compressor unloads, the condenser’s capacity increases and additional flooding is required. Using the same roof-top unit as in the earlier example (40°F evaporator and minus 20°F minimum ambient), a multiplier of .79 is shown in Table 2. And since we have unloaders (33-1/3%), this .79 is used to enter Table 3 to find a multiplier of .95. This final multiplier is applied to the 119 pounds calculated earlier to arrive at the final extra charge requirement: 119 x .95 = 113 pounds. This is added to the normal system charge to arrive at a total system charge.

Since the majority of “low and medium suction” condensing units are already flooded 75% or more for any minimum ambient temperatures below 20°F, no data is supplied for these units even when they use unloaders. The normal procedure is to recommend flooding from 90 to 100% for these units when they have unloaders.


Refrigerant charging procedures

Normally this information is supplied by the equipment manufacturer. And when it is available, it should be followed. When it is not available from the equipment manufacturer, the following suggestions are recommended.

Once the amount of extra refrigerant charge is calculated, care must be taken in charging the system to ensure the proper total amount of refrigerant getting into the system. This is especially true if the ambient temperature is below 70°F and the liquid drain valve (ORI, OROA, or LAC) is throttling the refrigerant flow from the condenser. A step-by-step procedure is given below for the two possible situations that can exist. And depending on the ambient temperature at the time the system is charged, each should be carefully followed to ensure proper system operation in both summer and winter. In either case, a liquid seal must be established in the receiver before the system can start to function correctly.

NOTE: While charging any system with head pressure control, the outdoor ambient temperature must be known. And if the system has compressor unloaders, it is important to know if they are functioning during the charging procedure. To keep this procedure as simple as possible, it is recommended that the unloaders be locked out (compressor fully loaded) during charging.


Charging of Systems with Parker Sporlan Head Pressure Control in Ambients ABOVE 70°F (After normal evacuation procedures)

Before Starting System

1. Connect refrigerant cylinder to a charging or gauge port on the receiver outlet valve.

2. Open the receiver valve approximately one-half way (so receiver and liquid line are connected to charging or gauge port).

3. Charge liquid refrigerant into the high side of the system. Weighing the charge is recommended with the initial charge consisting of approximately 2.5 pounds per system ton.

4. Remove the refrigerant drum and connect it to the suction side of the compressor. 

5. Charge refrigerant vapor into the low side until the pressure is above atmospheric pressure. Do not admit liquid refrigerant into the low side.

6. Start the system. 

7. Observe See•All moisture and liquid indicator (at receiver outlet) to see if the system is properly charged for the normal refrigeration cycle. CAUTION: Bubbles in the See•All can be caused by flashing due to pressure drop from pipe or accessory losses, etc. 

8. If the See•All shows bubbles, more refrigerant should be added, while allowing sufficient time for the refrigerant to stabilize and clear the See•All.

9. The extra refrigerant charge for head pressure control should be weighed in now by admitting liquid refrigerant to the high side. 


Charging of Systems with Sporlan Head Pressure Control in Ambients BELOW 70°F (After normal evacuation procedures)

NOTE: When charging in ambients below 70°F the procedure is very critical. Be sure to adhere to the following steps without fail. Failure to do so will result in overcharging the system.

1. Follow instructions 1 through 7 above. 

2. If the ORI, OROA, or LAC valve setting is correct for the system being charged, it is quite likely that some refrigerant will be backed up into the condenser and the See•All will indicate bubbles in the liquid line. 

3. Add more refrigerant, while allowing sufficient time for the refrigerant to stabilize and clear the See•All. 

4. At this point, the system is correctly charged for this type of head pressure control at the ambient temperature that exists while the charging procedure is taking place. 

5. If the system is designed to operate at ambients below the ambient that exists during charging, an additional charge will have to be added now. 

6. To calculate the additional charge required, follow the examples outlined under “Refrigerant Charge” except remember that the “head pressure control charge” is partially charged already. Refer to Tables 2 and 3. The difference in percentages between the minimum design ambient temperature and the ambient temperature at the time the system is charged gives the percent of extra charge still needed in the system. E.g., if this system was charged at an ambient of 50°F, we have approximately 40% of the extra charge in the system. This holds true as long as the compressor unloaders were not operating during charging. Therefore, the additional charge required is 95 minus 40 or 55% of the total extra charge calculated previously. This is .55 x 119 or 65 pounds.

Since good system performance during low ambient operation depends on proper refrigerant charge, it is very important that this phase of the installation procedure be done carefully. Many times, poor system performance will be due to too little or too much charge. And in many cases, this will be the last item suspected.


Jason Forshee

HVACR Tech Tip Article contributed by Jason Forshee, application engineer,  Sporlan Division of Parker Hannifin






Additional resources for you:

HVACR Tech Tip: What You Need to Know About Flooded Head Pressure Control

HVACR Tech Tip: Guidelines for How to Size Solenoid Valves for Split Condensers

HVACR Tech Tip: Guide to Servicing Blended Refrigerants


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