Improving Cryogenic System Performance
One of the most common methods to improve a cryogenic piping system is to reduce the amount of heat transfer into the system. Cryogenic liquid piping systems can be insulated in a variety of different methods from foam covered copper pipe to vacuum jacketed pipe. The latter is most efficient and uses technology similar to a vacuum jacketed thermos bottle. It should also be noted there are different levels of performance even within vacuum jacketed pipe or hoses depending upon vacuum and insulation quality. Fittings and connections along the way have to be considered as well. Non-vacuum jacketed valves and small sections of un-insulated pipe can add a surprising amount of heat transfer into a system creating huge problems.
A second method to improve the pipe system is by reducing restrictions to flow so there is less pressure drop in the system. While this can seem like the best solution, there is often a price/performance tradeoff and in some cases a longer start up time from shutdown to get the liquid to the point of use.
Another method to improve pipe system performance is to consider cool down time. During start up, cryogenic piping systems go from all vapor flow to nearly all liquid flow. This change happens as the pipe cools. The less mass in the pipe system, the more quickly the cool down occurs. Therefore, the pipe system should be sized for the best cool down time while remaining strong and providing low pressure drop.
Lastly, vapor can be mechanically removed from the liquid by means of vapor vents and phase separators placed along the flow path. Cryogenic equipment, like some phase separators discussed in the next section, effectively accomplish this.
Separating Vapor from the Liquid
Vapor can be taken out of a cryogenic system by using a gas vent (or vapor vent). These devices, installed at a high point or end of a system, have an internal float (see Figure 1) which lets gas out but once it encounters liquid shuts the vent line off. These are quite effective and simple in operation.
Figure 1: Gas Vent Diagram
Another device that can be used is a cryogenic phase separator. There are two common kinds used in industry:
- A simple kind that is nothing more than a sintered metal filter that lets gas pass through and liquid backs up behind the filter.
- A more complex phase separator that can deliver higher flows. These take the incoming two phase flow and put it into a tank that lets the gas escape from the top and the liquid from the bottom. This type of system, when properly controlled, can lower the fluid temperature by changing the pressure/saturation point – thus reducing the two-phase flow by changing system conditions. This is similar to a pressurized radiator cap on a automotive cooling system, although more complex since in this case the cryogenic liquid is being consumed by the process.