Frequently Asked Questions - Selection
The designer and installing contractor must ensure proper oil return from the evaporator coil in the system. All forms of modulation that change the velocity of the suction gas have this concern. We insure this is not an issue by maintaining a minimum velocity up the risers.
The APR Controls are classified by refrigerant and the maximum amount of modulation. Thus an APR-410-1 gives 1.5 tons of modulation on an R-410A system. Please note that the capacity listed on the APR is the modulation capacity not the system capacity. 100% modulation is NEVER acceptable. Example: a 3.5 ton system cannot use the APR-410-3 (3.5 tons of modulation).
The APR Controls are classified by refrigerant and the maximum amount of modulation. Thus an APR-410-1 gives 1.5 tons of modulation on an R-410A system. Please note that the capacity listed on the APR is the modulation capacity not the system capacity. 100% modulation is NEVER acceptable. Example: a 3.5 ton system could not use the APR-410-3 (3.5 tons of modulation).
You need to know the following, and if we are to assist, we will need to know, the following information; (we strongly recommend that you consult Rawal Devices engineers the first time):
- Refrigerant Type
- Total Systems capacity in tons
- Capacity of the lead stage in tons
- The compressor and circuiting configuration
Example: Dual stage/dual circuit (two independently controlled circuits)
Example: Dual stage/single circuit (two independently controlled stages on a single circuit)
- Is there a lead-lag control (does first stage on rotate between compressors/circuits)?
- Evaporator Coil Configuration
- Single circuit
- Dual circuit intertwined
- Dual circuit split face
- Or Other Configuration
- Split or Package System
- Split systems will require additional information, including:
- Suction line size (inches OD)
- Line size of ANY AND ALL suction risers (inches OD)
- Total suction line length
- Total length of ALL suction risers
The suction line size and more importantly is the riser size will determine if the oil will return. ANYTIME the suction line creates a trap or has a vertical rise (in direction of gas flow) there needs to be both velocity and volume to carry the oil up. WE ENSURE THIS IS ADDRESSED WITH PROPER SELECTION AND SIZING.
You will need to know total tonnage of the unit. Then the tonnage of the circuit and in particular the lead circuit (referred to as: First On/Last Off circuit). Then we need to know the tonnage of the lead compressor stage. Only one APR Control can be installed per circuit. In most cases, only one APR is needed per system. There are cases, however, where an APR Control on each circuit might be beneficial…split face evaporator coils, for example.
In most applications the installation of the APR on the lead circuit is enough. When both stages are operating, a drop in suction pressure will modulate the lead circuit. When the second stage shuts off, the APR Control will continue to modulate. In some applications of high humidity combined with 100% O.A. the use of an APR on both circuits may be beneficial.
In some instances the need for lead-lag control is lessened when using the APR Control. The number of hard starts of the first stage and also between stages is dramatically decreased when using the APR modulation Control. If lead-lag must be used, then an APR must be installed on each circuit.
Circuiting and staging can impact coil performance and system capacity. With a split face coil, for example, during first stage operation only half of the coil is active. This results in some coil-face air bypass over the inactive, often dry, portion of the coil. Please consult the factory with your specific situation.
The simple answer is the APR Control unloads the compressor and hot gas bypass loads the evaporator. THE APR CONTROL IS COMPATIBLE WITH MICRO-CHANNEL CONDENSOR COILS WHILE STANDARD HOT GAS BYPASS TO THE DISTRIBUTOR OF THE EVAPORATOR COIL, IS NOT AN ACCEPTABLE OPTION FOR MICRO CHANNEL COILS. THE APR CONTROL IS ACCEPTED BY ASHRAE 90.1 STANDARDS AND HOT GAS BYPASS IS NOT.