Let’s look at an example where the screw compressor will discharge directly into two reciprocating machines in parallel. The screw compressor will be sized to meet the combined flow requirements of the piston machines at a fixed suction pressure range. Let’s look at the same example we looked at previously. We will use a suction pressure of 25 psia, a discharge pressure of 75 psia and a flow rate will be 10 MMSCFD. This means the reciprocating machines will provide a total flow of 10 MM at 75 psia suction pressure, or 5 MM per machine. In this application we will typically control the capacity of the screw compressor by monitoring the discharge pressure. In the event that one of the reciprocating machines is shut down, the screw compressor will need to unload or reduce the flow rate by 50% to prevent the reciprocating machine from overloading the engine. The discharge pressure sensor will sense an increase in the discharge pressure and send a signal back to the control system telling it to unload the machine. Lube oil from the system will be directed through a four way directional valve to the backside of the hydraulic piston, forcing it to move the slide valve towards the discharge end of the machine. This will reduce the throughput of gas in the compressor until it reaches the desired rate of 5 MM.
Once the down stream compressor is started up again, the discharge pressure will begin to fall as the reciprocating machines take away the gas. The pressure sensor will provide a signal to load the machine and oil will be pumped through another port on the directional valve back to the other side of the piston, forcing the slide valve towards the suction end until the desired pressure is reached. If you are unsure of where the hydraulic cylinder is located, please refer back to Figure 2 earlier in the paper.
Operating at half capacity as we saw in the above example provides significant power savings as well. Figure 13 below shows the typical part load power consumption of a screw compressor at various different compression ratios. At low ratios, the offload power consumption is better than the high ratio machines. These power savings are significantly higher than reciprocating machines.
Lube Oil System
As we mentioned at the beginning of this paper, the machines we are looking at are oil flooded compressors. These machines require large amounts of lube oil to provide sealing between the rotor lobes and the casing, and the male and female lobes where the compression occurs. The oil is also required for lubrication of the bearings and shaft seals, and to reduce the heat of compression in the machine.
The lube oil system on a screw compressor is a closed loop system. The oil is injected into the machine in several places. The main oil injection port feeds the rotors directly with smaller lines feeding various points on the machine for seals and bearings. Once the oil is injected, passages within the machine will drain all the bearing and seal oil into the rotors where it combines with the gas. The gas and oil mixture is then discharged out of the machine. The oil that is injected must be removed from the gas down stream of the compressor.
A separate oil separator vessel is required to remove this oil from the gas. This vessel can be either vertical or horizontal in design, depending on equipment layouts and availability of space. The vessel will require coalescing type elements to remove as much oil as possible. Typical oil carry over rates from the separator are in the 10 ppm range. The oil separator also acts as a reservoir for the lube oil system. The lube oil will flow from the bottom of the separator, through an oil cooler where it is cooled from discharge temperature down to 140-160oF, through an oil filter and then back to the machine. Depending on the compressor manufacturer and the operating conditions, some machines require lube oil pumps to circulate the oil. Other manufacturers will use the differential pressure from discharge to suction to move the oil around the system.
The oil cooling can be done using two different methods. The direct cooling method simply uses a section in the after cooler to cool the oil using the ambient air. The indirect method cools the oil in a shell and tube or plate and frame cooler. A water/glycol mixture is pumped through the other side of the exchanger and then circulated through a section in the gas after cooler. This method requires an additional exchanger and water pump.
There are various arguments supporting each method. We have found the indirect cooling to be the most useful in our applications. It is particularly important to use this method in areas where cold ambient temperatures occur and the units are provided in heated buildings rather than outside. Here the oil is kept inside the building and not exposed to freezing temperatures. Direct cooling methods are not effective in cooler climates. Any oil left in an aerial cooler in cold weather during shut down periods will become more viscous and difficult to move around the system. In areas where the temperatures are warmer, direct cooling methods are seen more often. In these applications, vendors much choose the most effective and reliable methods for their clients. Although additional oil coolers and water pumps are required on the indirect systems, the cost of the aerial cooler increases with direct cooling as oil requires more surface area in the cooler than a water/glycol mixture. Another factor we need to consider is the amount of oil in the system. The direct cooling method will require much more lube oil than the indirect system. The oil used in these systems is typically a synthetic product that is much more expensive than a water/glycol mixture.
