Diffusion pump oil is boiled into a vapor in the boiler section of the pump (see figure 1) and accelerated to sonic speed (as a vapor) through precisely controlled jets.
Figure 1. Boiler section on a DIJ20 Diffusion Pump. These jets are designed such that they produce an oil curtain that covers the entire diameter of the pump (see figure 2).
Figure 2. Oil Vapor “Curtain”
Usually modern diffusion pumps have four stages or five stages of compression, which means four or five separate oil curtains in series. Each stage captures gas above the oil curtain and expels it below the oil curtain, and each subsequent stage has jets that have progressively shorter curtains. A longer curtain provides a great deal of surface area to grab molecules above it, but at the expense of curtain strength. Imagine you have your thumb over the end of a garden hose creating a spray. You notice that the water remains in a sheet for some distance before it breaks up into small droplets. The same is true of the oil curtains in a diffusion pump. If the curtain breaks up, some of the gas below it will blow backward, creating a loss of pumping. Thus, as the pressure gets higher through the stages of compression, the curtains get shorter and shorter. In a five-stage pump, the last stage is a high volume, high velocity stream of vapor that entrains the pumped gases to the exhaust. The fifth stage reduces the back pressure on each of the four stages above it, which helps the oil curtain retain its integrity with higher pump inlet pressures.
Figure 3. Five Stages of Compression
Diffusion pumps have two primary factors that make the pump work: heat and internal geometry. It has a very simple operating principle but can be very challenging to design and manufacture due to the tight constraints these two factors work under. Tighter jet nozzles will cause the pump boiler to work at higher pressure and temperature, which can create a stronger oil curtain, but can make it more prone to turbulence and loss of performance if too much power is fed into the boiler by the heaters. Wider clearances on the jets can make the boiler run at lower pressure and temperature but can take more power to drive higher oil vapor flow. They are excellent at pumping most gases, though they struggle to pump water vapor. Some factors that affect the pumps performance are:
- Inlet size: Far and away the primary driver of pumping speed in all high vacuum pumps.
- Oil type: There are a number of different specialized oils that affect the ultimate vacuum and performance in different applications. Compare the different types of oil
- Cooling water: Cooling water is part of the heat cycle of this pump. You need to add heat to vaporize the oil and then remove heat to re-condense it for the next cycle through the pump. Adequate cooling to the pump can make or break the operation.
Pumping speed compared to throughput.
Pumping speed is the pump performance number that most people will see published by diffusion pump manufacturers. It is a volumetric speed, which means that as pressure changes, the amount of actual gas molecules being pumped changes proportionally. In other words, the pump takes the same size gulp every second, but with few gas molecules present at lower pressures, each gulp is less concentrated. Throughput, on the other hand is a mass flow rate. It is a measure of the number of gas molecules moved from the inlet of the pump to the exhaust of the pump in each second. A pump with higher throughput will always pump a chamber down faster because it pumps more at higher pressures, where there is more gas to remove. Thus, when looking at specifications to determine the best pump for your system, make sure to compare throughput.
Actual pumping speed vs published pumping speed.
Quite a number of decades ago, the American Vacuum Society created a standard for measuring pumping speed of diffusion pumps. Unfortunately, the method behind this standard calculates a pumping speed that is about 70% higher than the actual real world pumping speed. Thus, all diffusion pumping speeds that use the AVS standard are overstated by close to a factor of 1.7. Later, an international standard was created to measure diffusion pumping speed, the ISO (International Standards Organization). This new standard by ISO is much closer to reality, though still about 25% overstated to actual real world pumping speed. The reason behind the variance to the ISO standard is that measurements done for this standard are done in a laboratory with a single gas like nitrogen under laboratory environmental conditions. In the real world, the chambers we are pumping are filled usually with atmospheric air which contain many gases including water vapor, which is challenging for most vacuum pumps to remove quickly. If you have a high incidence of water vapor in your location, the best pump is a cryogenic pump, which uses the condensable nature of the water vapor to its advantage by freezing the molecules onto huge surface areas.