Welcome to the FAQ Section of Rawal Devices

As the heat content of return air drops, the suction pressure drops opening the APR Control. The hot gas gets sent into the desuperheat chamber then back to the suction line. A liquid injection hot gas valve mixes liquid with the HG (in the desuperheat chamber) when the mixed gas temperature rises to 60F.

The APR Control derates the evaporator coil capacity extending the run time while keeping the active portion of the coil at a constant temperature (below dewpoint) thereby dehumidifying during the extended run period. Watch the video below for more information.

Before you call Rawal Devices, we will need the following basic information; including equipment information, equipment cycle times, general building and space information, what you believe is possibly causing the humidity issue, and outdoor climate. It is likely that the APR Control could provide a simple and cost effective solution.

Often systems will short cycle either as a result of being oversized from the beginning, or simply because the load varies too much during the air conditioning operating season. The APR Control will often be employed to help reduce system capacity and extend runtimes, but the installer must confirm that suction pressure will fall far enough to cause the APR Control to begin to modulate the system capacity. If load does not fall enough, the APR Control may not be helpful in that specific application. Please refer to adjusting the APR Control, in the Start-up section of this FAQ.

APR Control will keep the suction pressure from falling too far and causing the coil temperature to fall to a point where it will start to frost or freeze.

The APR Control can protect against the problems related to clogged filters, which will, for example, cause airflow across the evaporator coil to be reduced causing a drop in suction pressure which will eventually result in coil frosting. The APR will maintain suction pressure by modulating refrigerant flow reducing capacity at the evaporator. Proper and regular equipment maintenance will eliminate this problem.

High humidity conditions, in spaces serviced by direct expansion equipment can result from short cycling conditions, or as a result of too much cooling capacity compared to load. Studies have found that if a short cycling condition exists, and the evaporator coil is not kept “active” for a long enough runtime period, the coil may start to load with moisture, but runtime could be too short for that moisture to fall to the drain pan and down the drain…which is officially when dehumidification is completed, all other conditions may result in the moisture re-evaporating back into the air stream and into the space!

Could be the result of the system not staying on long enough, staying off too long, or even something as simple as a problem with infiltration (or the building envelope).

If the system has an APR Control but is not staying “on” long enough. Simple adjustment of the APR Control’s hot gas valve may be all that is necessary. By increasing the pressure setpoint of the valve, meaning at what suction pressure the valve begins to open, you should be able to increase the runtime significantly.

Understanding the cycling issue your particular system is experiencing is critical to making the APR Control suitable to your application. If you have ever spoken with Rawal Devices Engineers, you will recall that we have a saying “If you have short on/off cycle condition – the APR Control is usually an excellent solution! If you have short “on” cycles with long “off” cycles, the APR Control alone may not resolve the high humidity condition, and additional strategies for shortening the “off” cycle may need to be utilized. For example, a system that has a cycle time of 5 minutes “on”/ 5 minutes “off” is easily corrected with the APR Control alone, as the APR Control will extend the “on” portion of the cycle to meet the minimum necessary for real dehumidification. A system that has a cycle time of 5 minutes “on” but 30 minutes off may need some help. A humidistat may be helpful in driving the system on when there is not sufficient sensible temperature in the space to cause a standard thermostat ON; or if available, reheat (also driven by a humidistat) could further improve humidity control performance. The APR Control will usually be all that is necessary, but these concepts should be kept in mind.

The system has an APR Control but is staying off too long, as a result we are unable to maintain humidity levels in the conditioned space. Systems that have this problem may not be correctable with the APR Control alone. The APR Control can extend runtime “on” cycle, but cannot change the off cycle.

Understanding the cycling issue your particular system is experiencing is critical to making the APR Control suitable to your application. If you have ever spoken with Rawal Devices Engineers, you will recall that we have a saying “If you have short on/off cycle condition – the APR Control is usually an excellent solution! If you have short “on” cycles with long “off” cycles, the APR Control alone may not resolve the high humidity condition, and additional strategies for shortening the “off” cycle may need to be utilized. For example, a system that has a cycle time of 5 minutes “on”/ 5 minutes “off” is easily corrected with the APR Control alone, as the APR Control will extend the “on” portion of the cycle to meet the minimum necessary for real dehumidification. A system that has a cycle time of 5 minutes “on” but 30 minutes off may need some help. A humidistat may be helpful in driving the system on when there is not sufficient sensible temperature in the space to cause a standard thermostat ON; or if available, reheat (also driven by a humidistat) could further improve humidity control performance. The APR Control will usually be all that is necessary, but these concepts should be kept in mind.

Undersized ductwork will lead to reduced airflow across the evaporator coil. The APR Control will protect the refrigeration circuit from an extreme suction pressure drop that will normally result. The APR reduces refrigerant flow to eliminate the chance of coil freezing as a result of low evaporator temperature. Please note that system capacity may be reduced to a point where the system is inadequate to serve the space – the APR Control will protect the circuit and allow the system to function, but a full evaluation of the impact should be made.

Improper fan size, or undersized evaporator coils (i.e. mismatched equipment) can be temporarily relieved with the addition of the APR Control. While this should not be the first option, the APR Control will manage refrigerant flow based on the limit of the specific APR Control model to better match the output of the condenser section with the capacity of the evaporator/fan coil.

Liquid slugging can result in long term damage to the compressor. The liquid slugging is usually the result of low load conditions. The APR Control can manage refrigerant flow into the evaporator coil, modulating condenser output so that a low suction pressure condition (and as a result, liquid slugging) will not occur.

As the APR Control can help match system capacity, it is, by extension, minimizing extremes in space temperature and relative humidity. By simply stabilizing space conditions with the APR Control, a significant number of “nuisance calls” can be eliminated. Also, by reducing the wear and tear related to short cycling, coil frosting and liquid slugging, it is likely that adding the APR Control to the circuit will reduce downtime over the life of the system.

High altitude applications were an interesting discovery for us… (We had never worked in high elevation environments!). Low air density results in inadequate heat transfer at the evaporator coil, leading to low suction pressure and sometimes a coil icing condition. The APR Control will maintain suction pressure above a minimum psi as set on the APR Control (110 psi w/ R-410A when shipped from the Rawal factory).

Whether on multiple zone VAV applications or simply where a VFD has been added to evaporator fan of a single zone system, the APR Control will modulate the capacity of the system to match the changing load. By responding to changes in suction pressure, the APR will reduce system capacity as load on the system is reduced (via zoning or simply a VFD) putting the system in to balance so risks associated with low airflow associated with VAV systems are reduced, if not eliminated.

Often budget constraints force a less than perfect solution to be used in High Percentage and 100% Outside Air applications. By delivering a system with improved runtime capability and the simple capacity modulation to match the load of intake air, there will be less extremes in space conditions than with a single (or even dual) stage system. Keep in mind the benefits of using a deep (high latent capacity) evaporator coil will provide with or without the APR Control…mostly with!

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.

Yes.

Yes, you will need at least three “T’s” or someway of connecting the APR Control to the system suction line, liquid line, hot gas line and the external equalizer line where necessary. In addition, isolation ball valves are required. Without the ability to isolate the APR Control from the circuit there may be NO WAY TO CHARGE OR CONDUCT SYSTEM DIAGNOSTICS.

Yes, vibration resulting from the operation of the compressor could put strains on the piping and control. A simple mount to either the cabinet or a stand is all that is needed.

Yes, the APR Control can only be mounted in such a way that the Schrader/access valve is located down and the mixed gas discharge to the suction outlet up. ANY OTHER POSITION OR ORIENTATION IS UNACCEPTABLE AND WILL LEAD TO SYSTEM PROBLEMS AND LIKELY DAMAGE.

No, it is preferred to install the APR inside the unit housing, but it is also ideal to have access to the control. If exterior installation is preferred make sure there is limited access to the unit from the general public.

No, the chamber holds no charge.

This is a delicate control much like a TXV or any other device added to the refrigerant system. Thus you need to provide the protection from excessive heat when soldering or brazing.

We recommend Harris Products Stay-Silv 15 or equivalent 15% silver alloy for situations in which close fit-up does not exist, and where thermal expansion and service vibration are involved.

Please note that the APR Control valves that will be connected are of stainless steel construction, but the interior of the branch connection is copper clad to allow for an easier process.

Yes for all three lines. The liquid line needs to be connected after the system dryer (only if the system already has one. We are looking at the filter part of the dryer to protect the desuperheater valve. When connecting the hot gas line you need to ensure no oil traps are formed at the connection or could cause the APR chamber to trap oil. Do not connect into the bottom of a horizontal hot gas line, A potential oil trap could be formed. Installation requires connection into the side or top of the horizontal line. The same is true with the suction line with one critical addition, because the velocity in the suction line is lower (than the hot gas line) the connection into it needs to be from the top (reference of 10-2 on a clock), even connecting into the side could cause some issues pertaining to oil.

Bulbs connected to the APR Control valves will be installed between the Tee connecting the APR to the suction line and the compressor. These bulbs must be strapped to the suction line and properly insulated. Equalizer Lines (where the APR Control has such connections) will tap into the suction line between the suction line Tee and the compressor.

Yes, This is as critical as any step in the installation of the APR control. The insulation needs to properly installed and completely covering the APR Control valve bulbs. The insulation needs to be adequate to protect the bulb from the ambient temperature.

No, the APR Control must not be insulated as this would reduce the effectiveness of the APR Control’s operation. System heat (in the form of hot gas) is shed through the wall of the desuperheater chamber.

The first question is what type of low ambient control do you have? If it is a modulation type of either the fan motor or some type of damper control for the air then there should be no issue. As long as we have somewhat of a steady head pressure the APR Control can operate just fine in the system. Low ambient control with simple fan cycling is less than ideal. This type of low ambient control produces significant and erratic changes in head pressure. This can cause issues with operation of the APR Control since it operates off of suction pressure. If a head master type of control on the refrigeration circuit is used then a solenoid with a pressure sensor is needed to lock out the control under low head pressure. This will allow for the system to stabilize before the control is brought into operation.

Make sure the APR Control was open to the circuit during evacuation. Prior to charging the system you should isolate the APR Control out of the system with the three isolation valves (closed). You will need to have the APR Control isolated during the charging of the system and the system should be run initially with the APR Control entirely out of the circuit (isolation valves remain closed). This is necessary to make sure the charge is correct and that the air conditioning system is operating as originally designed, and so there is a baseline to measure the system “with the APR Control modulating” against. In addition, there is the benefit of allowing the system filter/dryer to clean the refrigerant before it is allowed to enter the APR Control. This helps ensures for the cleanest startup possible.

The APR Control is shipped from the Factory set for approximately 40 degrees F. evaporator coil temperature, or to start opening at 118psi (in an R-410A system). Most application will not require adjusting the APR Control. But if the runtime is inadequate or low load operation does not cause suction pressure to fall low enough, you may need to adjust the APR Control hot gas valve. Please call Rawal Devices Engineer’s (800-727-6447) at the factory prior to attempting any adjustment.

No, NORMALLY YOU WILL NEVER TOUCH THE INJECTION VALVE. If you believe there is any need to adjust the desuperheating injection valve, please call Rawal Devices Engineer’s (800-727-6447) at the factory prior to attempting any adjustment.

You will not be able to judge what the unit is doing when an APR control is operating. You will need to isolate the control from the system with either a ball valve or solenoid valve (with manual stem) in the suction line between the outlet of the control and the “T” on the suction line. We recommend that isolation ball valves be installed on all connections to the APR Control device. Without these isolation valves the APR Control can mask several types of issues.

The APR Control is shipped from the Factory set for approximately 40 degrees F. evaporator coil temperature, or to start opening at 118psi (in an R-410A system). Most application will not require adjusting the APR Control. But if the runtime is inadequate or low load operation does not cause suction pressure to fall low enough, you may need to adjust the APR Control hot gas valve. Please call Rawal Devices Engineer’s (800-727-6447) at the factory prior to attempting any adjustment.

The bellows in the Hot Gas portion of the control will only withstand a certain amount of pressure. There are a number of causes for high pressure spikes in a system. If the system doesn’t have some type of head pressure control the compressor can operate above the safe limits of the APR Control. Normally safeties are in place to prevent this (excessive bypass) if they haven’t been compromised. High head pressure is caused by the following; overcharged, dirty or plugged condenser coils, non-condensables in the system and the loss of air flow. Whether air cooled or water cooled unit, anytime you lose the cooling medium the head pressure will rise. ANYTIME A HIGH HEAD PRESSURE OCCURANCE HAS HAPPENED THE CONTROL NEEDS TO BE CHECKED. THIS IS AN ABNORMAL CONDITION IN THE SYSTEM AND MANY COMPONENTS MAY BE AFFECTED.

First isolate the APR Control to see if you have an issue with system charge, evaporator expansion device, or some other system issue. If the system appears to be operating properly with the APR Control out of the circuit, then you will need to turn you attention to the control. Open isolation valves and allow the unit to go into deep modulation (usually best accomplished by reducing airflow across the evaporator coil). Check the compressor superheat at the desuperheater bulb (Is the desuperheater bulb located correctly?). The temperature of the suction gas entering the compressor should be maintained below 65 degrees F. If not, you may have an issue with the desuperheating injection valve. If the problem persists consult with engineers at Rawal Devices.

If you have checked all things related to the system such as condenser coil (clean?), charge, and other general maintenance items, then the first thing to check is the insulation on the bulb(s) from the control, most particularly the injection valve. The bulb insulation may have deteriorated or maybe insufficient for the ambient conditions you are dealing with. APR Control injection valve bulb must sense suction line temperature (between the APR Control mixed gas discharge tee into the suction line and the compressor) and not ambient.