Frequently Asked Questions –
Application


 

How does the APR Control work?

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.

I have a humidity issue what do I do?

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.

The air conditioning system is Short Cycling – how can the APR help?

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.

How does the APR Control protect the system?

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.

How does the APR Control help with lack of maintenance problems such as clogged Filters?

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.

The air conditioning system is causing my space to have Humidity Issues: How can the APR Control help?

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!

We are getting too much fluctuation in temperature and humidity levels, how can the APR Control help?

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).

My ON cycle is too short and we are unable to reduce relative humidity levels, how can the APR Control help?

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.

My OFF cycle is too long, how can the APR Control help?

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.

I was told that the system has undersized ductwork: can the APR Control help?

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.

The system has improper Fan (or evaporator) Sizing: Can the APR Control help?

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.

The air conditioning system has a Liquid Slugging problem – what can the APR Control do?

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.

Can the APR Control be used to reduce downtime and Service Calls?

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.

I have a High Altitude installation: Can the APR Control help?

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).

How is the APR Control used in VAV System enhancements?

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.

How is the APR Control used for systems with High Percentage Outside Air?

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!