Explore our step-by-step guide for selecting the perfect VRF AC system for your needs, whether it’s for your home, office, hotel, or any other application. VRF AC system nowadays is the most widely opted air-conditioning solution for almost all kinds diverse applications like residences, small and big offices, hotels, hospitals, industrial, educational, etc. It is no longer considered a luxury or a one-time buy. The end–user today is typically a second or third time – sometimes even a multiple time user. In this scenario, the users are quite well-versed with VRF AC System technology and behavior. However, they are generally not aware of optimum system selection. The selected system however could either be under-rated or over-rated. An under-rated VRF AC system consumes more power and provides poor performance resulting in lower efficiency. An over-rated VRF AC system consumes more power than the anticipated consumption along with higher capital expenditure on the system and ancillary electrical items. In this scenario, the VRF specialist must add value that end users are willing to accept before the product or service becomes an issue.
Three Phase Approach for selecting a VRF AC System
For adding value to the product or service, a VRF specialist can adopt a Three Phase Approach.
We must understand that end user is the prime stakeholder of the system, and must be educated about the VRF AC system during the diagnosis and collaboration phases.
A. Design Conditions
There can be a difference between the expectation of the end user and how a VRF specialist designs the system. To deliver the optimum VRF solution, the end user must be interviewed for the following:
A.1 Selection of Indoor Temperature
The VRF specialist must enquire from the end user about the requirement of indoor temperature, and needs to check whether the demand is normal or abnormal. A very low indoor temperature requirement leads to an over-rated VRF AC system. One must educate the customer the advantages of selecting a normal temperature, i.e. 22°C to 27°C, for thermal comfort.
A.2 Selection of Outdoor Temperature
If the machine is selected for the highest outdoor temperature, which would prevail only for a few days of the year, it will result in higher than optimum outdoor unit capacity, higher power consumption and higher electrical ancillary investment. Instead, the end user must be informed about the average high temperature of the location in order to derive the optimum outdoor capacity.
A.3 Type of Indoor Unit
The VRF specialist must not assume the type of indoor unit on its own. One must enquire from the end user proactively the type of Indoor unit required, during the design stage. This plays an important role in the selection of optimum capacity of outdoor unit. One can select lower apparatus dew point (ADP) for non-ducted type of indoor units, and higher ADP for ducted type indoor units. For example, if the VRF specialist assumes a ducted unit for a particular room without enquiring from the end user, it would lead him to input a higher ADP during heat load estimation, which would lead to the requirement of higher air flow, resulting in the selection of higher capacity indoor unit as well as outdoor unit. But if the end user has indicated a ceiling suspended cassette or a wall mounted unit instead of a ducted type, a lower ADP selection will save the unnecessary increase in outdoor unit capacity.
A.4 Tentative VRF AC System Outdoor Unit Location
The benefit of understanding the tentative location of the outdoor unit enables the VRF specialist to derive the actual capacity of outdoor unit with due consideration decrease in capacity due to piping length. In order to derive the actual de-ration in capacity caused due to longer piping length, the VRF specialist should calculate the distance between the tentative farthest indoor unit and the tentative outdoor unit location. One must apply a correction factor on the proposed outdoor unit capacity to compensate for the reduction of outdoor unit capacity. This would lead to optimum selection of the outdoor unit.
Correction Factors
Correction Factors are basically multiplying factors required to be multiplied to the nominal outdoor capacity to get actual capacity of outdoor considering reduction in the capacity of outdoor unit due to High Ambient, Lower Indoor Conditions & Longer piping length. Primarily, VRF machines are designed to deliver nominal capacities. One must add correction factors.
Understanding the Air-conditioning Loads pertaining to VRF AC System
Above, we understood how to select Indoor Units of a typical VRF AC system for cooling purpose. Now, in order to understand designing or selecting a VRF Outdoor, one needs to get acquainted with Heat Load Estimation. Although Heat Load estimation considers geographical conditions of location, Building Material, Orientation of Building, Type of Glass Envelope etc., but these all things ultimately concludes to majorly two outputs i.e. Refrigeration Tonnage (Say Capacity required to Cool a place), Airflow Rate (Say amount of Air required to Cool the place at given point of time).
Actual Air-conditioning Load (Say To-be Conditioned Areas)
The cumulative of air-conditioning capacities of areas which are to be conditioned at any given moment of time.
Actual Diversified Load (Say Extra Areas)
The cumulative of air-conditioning capacities of areas which are extra to the capacity of outdoor unit but may be used when the occupant shifts from “To-be conditioned areas” to “Extra Areas”.
Total Refrigeration Load
It is as simple as cumulative of air-conditioning capacities of “To-be conditioned areas” & “Extra Areas”.
Total Connected Load
Connected Load is basically the sum of capacities of Indoor units which we select against the air-conditioning demand of areas based on Heat Load Estimation.
Understanding Diversity:
To understand diversity, a typical VRF AC system is meant to serve two sets of zone: the primary set of zones that are required to be air-conditioned on priority by the VRF AC system, and the secondary set of zones that are connected but will be used only when load shifts from the primary set of zones. The diversity, or total diversified load as defined earlier in this article, is not meant to be in use in parallel to the actual running load as it affects the VRF AC system’s performance. The industry erroneously uses the term diversity for combination ratio. Now, we have explained the difference. For example, let us assume a residence that has living and dining rooms, a master bed room, and two bed rooms. Ideally, the primary set of zones to be conditioned can be assumed as the master bed room and two bed rooms, and the secondary set of zones as the living and dining room. The living and dining room are considered to be the diversity. The VRF outdoor would feed the primary set of zones and, in case any room load shifts to dining and living room, the outdoor would cater to the diversified load. Total connected load comprises both the primary and the secondary set of zones. The actual running load with reference to the end user is the master bed room and two bed rooms. On the other hand, the end user can also prioritize the master bed room, one bed room and living room as the primary zone, and may consider the secondary zone as one bed room and dining area. It would be totally dependent on the user’s requirement as to what constitute the primary and secondary zones.
Step by Step Procedure to Select the VRF AC System:
Optimum selection of VRF (primarily for cooling application) is defined below.
Step 1
Estimate the heat load for the cooling application.
Step 2
Select the indoor unit (type and capacity) based on the client’s requirement and air flow required; most of the times the selected capacity will be higher in order to satisfy the air flow requirement.
Step 3
Now calculate or analyze the following:
1. Find the total refrigeration capacity of heat load estimation, say ‘A’.
2. Find the cumulative of capacity indexes of connected indoor units, i.e. connected load; convert this into the total capacity index number as defined earlier, say ‘B’.
3. Calculate or find the total actual running load ‘C’, based on the end user’s input.
4. Calculate or Find the Actual Diversified Load ‘D’, based on the end user’s input.
Step 4
Select the provisional outdoor capacity, say ‘E’, using the sum of capacity indexes of the selected indoor units, ensuring the combination ratio is between 50% and 130% (refer Annexure 1, wherein the value ‘B’ should fall between the minimum and maximum range of the cumulative of capacity indexes of connected indoor units:, column d and e are 50% and 130% combinations). The provisional outdoor capacity Index should be selected in such a way that it should fall in the bracket of minimum and maximum combinations.
Step 5
Check the Combination Ratio ‘CR’, i.e. ‘B’ divided by ‘E’. If it is within the 130% limit, proceed further; if not, repeat the selection process.
Step 6
Calculate the de-ration in the value ‘B’, i.e. cumulative of capacity indexes of connected indoor units. De-ration due to ambient temperature and indoor temperature conditions based on end user’s input, Say ‘X%’ and ‘Y%’ De-ration due to extended piping length between the outdoor unit and the farthest indoor unit, say ‘Z%’. Calculate the total reduction in capacity, say Z1 = X%+Y%+Z%.
Step 7
Derive the Actual Outdoor Unit Capacity, say ‘AC’ = Provisional Outdoor Capacity – Z1.
Step 8
Check whether ‘AC’, i.e. the Actual Outdoor Unit Capacity, is equal to or greater than the Actual Running Load ‘C’. If the corrected capacity is larger than or equal to the required Actual Running Load, the selection is complete. In the event that the corrected capacity is lower than the Actual Running Load ‘C’, the selection steps should be repeated from the point where the outdoor unit capacity is provisionally selected, i.e. Step 4. Refer sample calculation in Annexure.
Conclusion
This guide is meant to increase the ease of selection for VRF AC systems. It would help the VRF specialist select the VRF AC system based on a professional approach, and the end-user or the prospect would be able to independently verify the selection.
