The compressor requires a constant supply of hydraulic oil for lubrication during operation. Below are components tied to the hydraulic oil system,
Sump
The Sump has two primary fuctions.
The first, is the storage tank for the compressor hydraulic oil.
The 2nd, is the first of 2 stages in the separation of the hot hydraulic oil and hot compressed air.
The opposing screws type compressor requires hydraulic oil to keep the screws turning. As air is pulled in and compressed, it is mixed with the hydraulic oil. Only the air compresses. The oil does not. However, the compression generates heat that heats both the air and oil. The combined hot oil and air leave the compressor and head 1st to the Sump. The hot oil drops to the bottom of the tank like Sump while the hot air rises to the top. The air then heads towards the Air/Oil Separator. The hot oil heads to the the heat exchanger for cooling and reuse in the compressor.
The Sump may come in different shapes depending on the manufacturer and system model, however, they all have the same two functions.
The Sump also incorporates the Site Glass that allows the operator to both check the oil level prior to operations, and again periodically during operations.
Site Glass
The site glass is mounted on the Sump and gives a view into the sump to check the level of the hydraulic oil. It is recommended that the hydraulic oil level be maintained 1.2 way across the site glass. The oil level can drop (system use, torn AOS, frozen heat exchanger, etc.), or increase if water leaks into the oil (frozen heat exchanger, stuck check valve, etc.) Keeping it ½ way across the glass makes it easy to see changes. If the level is below the glass, how low is it? There is no way to tell if it is just below the glass or completely empty. The expensive compressor can only run a few seconds without hydraulic oil before it destroys itself by seizing up. Hydraulic oil is critical. If the oil level moves up above the glass line indicates the possibility of water mixing in with the oil. Diluted oil is also very bad for the compressor lubrication. A CAF operator can tell a lot and prevent big issues just by checking the site glass daily and periodically during CAF operations. Any changes should immediately be referred to you F-7 Certified EVT mechanic.
Temperature Sensor
The temperature sensor is mounted on the sump. It senses the hydraulic oil temperature and sends a signal to the pump panel mounted temperature gauge.
Temperature Gauge
Alarm Sytem
Large CAF systems are equipped with red light and beep sounds if the hydraulic oil temperature gets too high. The alarms are set (generally at 250° F) to sound in time for the operator to either correct the high heat issue or to place the system in unload mode before the system suffers damage. Failure to react to the alarms may result in catastrophic damage to the CAF system. The hydraulic oil flash point is 309° F. Be sure to read the manufacturer’s instructions regarding the availability and function of any system alarms.
WARNING: during compression, there is NOT a lot of time between 250 & 309°F. Always be attentive to the temperature gauge – recognize problems anytime the temperature begins to increase above normal operating temperature. DO NOT WAIT FOR THE ALARM TO SOUND!!!
Air/Oil Separator
The AOS is the 2nd stage in the separation of compressed air from the hydraulic oil (the first being the sump). While it looks like a larger oil filter, the cannister’s function is to filter out the hydraulic oil mist in the compressed air and return the oil to the compressor while the compressed air passes through the filter and heads on downline towards the hose line to mix with the solution to make CAF.
A tear in the AOS will allow oil to pass through the filter and on downline and out through the hose along with the CAF. This tear can be detected 2 ways – 1st – by regularly checking and operational monitoring the sump oil level (site glass). The oil level will drop. The 2nd method of detection is by placing your bare hand over the air only outlet during warm up checks. While some moisture is normal during the warmup check, oil is not. If the hand remains dry, oil is not passing by the AOS. If oil collects on your hand – immediately check the oil level, take the system out of service, and have your F-7 Certified EVT mechanic replace the AOS.
Heat Exchanger
Air compression generates heat. The heat exchanger is utilized to keep the hydraulic oil cool. Hydraulic is combustible and must remain well below its ignition temperature. There are several types of heat exchangers in use.
The first, a Shell & Tube is a cylinder-shaped device. Inside the cylinder is a coiled copper water line. The hot hydraulic oil passes through the heat exchanger cylinder surrounding the small water lines. A small amount of cool water from the fire pump circulates through the heat exchanger (small copper tubes) removing heat that is transferred from the hot hydraulic oil. The cooled oil then returns to the sump while the now hot water heads to the fire engine’s water tank to disperse the heat in the cooler water in the tank.
The 2nd type is a Brazed Plate that also utilizes water from the fire pump. This plate has tiny water ways and requires an extra fine mesh on the Water Line Clean Out Trap (Wye Strainer). It is critical that the clean out strainer be inspected often to prevent the stoppage of cool water flow.
Both the Shell & Tube and the Brazed Plate utilize small water lines and as such, are both susceptible to freezing. It is imperative that both types are properly drained of water during the cold freezing temperatures. A frozen heat exchanger must be replaced.
The 3rd type is air-oil cooling, operating similar to that of a car radiator. The air flowing over the fins removes the heat from the oil. The type of heat exchanger uses no water and is therefore not subject to freezing.
Wye Strainer (Water Line Cleanout Trap)
Covered in detail on the How NOT to Destroy a CAF System Page. To keep debris passing through the fire pump from circulating and possible plugging the small water lines of the heat exchanger, a clean out wye strainer or pump panel mounted basket strainer is added with a collector screen to catch such debris. If enough debris is collected, the screen trap can become plugged preventing water flow to the heat exchanger. This is the number one cause of hydraulic oil overheating. (A Very Bad Thing). With the pump off, the screen should regularly be inspected and cleaned as necessary.
A ball valve can be added to the screen side of the wye clean out strainer. The valve can be opened during pump operations – if the screen is not completely plugged, the pump pressure might blow the debris out of the ball valve.
Hydraulic Oil
Hydraulic is another component of the CAF System. It is necessary to keep the compressor lubricated. See “Hydraulic Oil” in the Maintenance section for details on the oil type.
Due to heat generated during air compression, it is critical that the CAFS operator continually monitor the hydraulic oil volume and temperature. (See How Not to Destroy a CAF System)
