Case study: Designing VRF for an office
Using an office building as an example, a variable refrigerant flow system is designed to meet the owner’s needs and current standards
Let’s apply the four design steps to the following example office building preliminary layout shown in Figure 5. This system is in a small office building with 10-foot-high ceilings using refrigerant 410A in the variable refrigerant flow system. The VRF system cooling capacity is 5 tons. Assume all rooms shown are offices.
For this example, we will use the rule-of-thumb of 6 pounds/ton to estimate a total refrigerant charge of 30 pounds. The ASHRAE Standard 15: Safety Standard for Refrigerant Systems and Designation and Classification of Refrigerants occupancy is commercial occupancy. From Standard 34 Table 4-2, we see that refrigerant 410A has a RCL of 26 pounds/1,000 cubic feet. Using Figure 1, the minimum allowable floor area is 115 square feet for a 30 pounds capacity system with 10-foot ceilings.
Review of Figure 5 finds that room 1 has a floor area of 100 square feet, which is below the minimum allowed floor area. Any space with a 10-foot ceiling that is below the minimum allowable floor area will exceed the refrigerant concentration limit for R-410A.
Now we will evaluate possible solutions to bring the system into compliance with the standards. Any of these options are acceptable solutions but the ultimate selection will most likely be based on the owner’s preference and budget.
Option 1 could be to raise the ceiling from a 10-foot to a 12-foot height. The new minimum allowable floor area would be 96 square feet for the raised ceiling. A ceiling move would need to be coordinate with the owner, architect and other engineering trades. This option is illustrated in Figure 6. Raising the ceiling may not be an option due to limited above ceiling space. Another solution to increase the floor area would be to install a ceiling grill or to undercut the doors.
Option 2 would be to revise the indoor fan coil unit selections to have a single fan coil unit serve Room 1 and Room 2. This would increase the floor area for the assigned fan coil to 395 feet, which is above the minimum allowable floor area. This option is illustrated in Figure 7. A disadvantage to this option is the loss of zone temperature controllability. Group spaces with similar heating, ventilation and air conditioning load requirements when implementing this option.
Option 3 could be to revise the indoor fan coil unit design to use ducted supply with plenum return fan coil units. Similar to option 2, this would change the room volume (ceiling height) of the space and lower the minimum allowable floor area of the system. The new minimum allowable floor area would be 89 square feet. This option is illustrated in Figure 8.