Twenty years ago, high-side piping was just “condenser piping,” although the necessity to purge noncondensibles was well understood. How it was to be accomplished was not very clear.
Fifteen years ago, thermosyphon oil cooling was gaining acceptance. Today it is the preferred and most commonly used method of compressor oil cooling. A great deal of attention has been given to individual component piping, although most attention should be given to how interconnecting piping can affect the performance of components.
Years ago, it was generally assumed that the highest pressure in a refrigeration system was at the compressor discharge. Now, thermosyphon oil cooling has changed this. The thermosyphon oil cooler on a screw compressor can, and will, generate the greatest system vapor pressure.
Consider that the normal inlet oil temperature is 175 to 180 degrees F (607- to 644-psi saturated ammonia [NH3] pressure), and outlet oil temperature is 120 to 140 degrees (300- to 394-psi saturated NH3 pressure). If a screw compressor started with the discharge valve closed, the compressor would shut down on high-pressure safety at 200 to 220 psi.
On the other hand, if the same compressor started with the thermosyphon cooler valve off, the thermosyphon cooler could generate 420 psi prior to high oil temperature cutout at 145 degrees. Under normal operating conditions, it is capable of generating 300 psi (120 degrees) based on a 15 degree approach to the leaving oil temperature.
Remember, a fluid always flows from some given pressure to a lower pressure. The pressures used in this article are very close to actual system pressures. Nevertheless, they are nominal and intended to explain the concept and procedure to use in analyzing high-side piping flow.
Follow the pressure drop in Figure 1.
For gas flow from points A to B, B to C, C to D, F to B, and H to I, the upstream pressure is greater than the downstream pressure. For liquid flow between points E to F and F to G, there is a pressure loss due to the flow of liquid. However, pressure generated by the static head of the liquid is greater than the pressure loss created by flow. Therefore, the downstream pressure is greater than upstream.
In this example, the direction of flow of each segment can be identified and all pressure losses can be accounted.
Let’s examine what happens if we reconfigure the liquid drain line from the condenser outlet to the top of the receiver. Use the same conditions and then reanalyze the pressure drops due to flow (Figure 2).
The heat load from the thermosyphon oil cooler will maintain the receiver pressure (point F) at 180.5 psi. This pressure is greater than the inlet and outlet condenser pressures. If the drain line is generously sized, gas will flow from the receiver to the condenser through the condenser outlet piping (point D, Figure 1).
With this configuration, the condenser outlet pressure will rise and still allow liquid to drain from the condenser to the receiver. This is commonly referred to as two-way flow.
SYSTEM PERFORMANCEA controversial question is, “How can these two piping methods affect system performance?”
AUTOMATIC PURGINGAutomatic purging requires refrigeration and is normally performed with the system in operation. The refrigeration for the purger is supplied by the system being purged.
Noncondensibles are always pushed by gas flow to the lowest pressure area in the high side. In Figure 1, this area is located at the outlet of the condenser (point D). Sample gas supplied to the purger flows through valve V3. In Figure 2, the lowest pressure area is located somewhere within or near the middle of the condenser (point C). Any foul gas is very difficult or impossible to remove — no connection is available to draw sample gas.
AUTOMATIC PURGING EFFICIENCYA trapped condenser outlet prevents noncondensibles from moving between the condenser and the receiver when pressure variations occur. Confining and concentrating noncondensibles permits an automatic purger to remove noncondensibles at a higher rate, because the sample gas flowing into the purger will have a higher concentration of noncondensibles.
The higher the concentration of noncondensibles, the less NH3 there is to condense out of the sample gas. This results in a lower refrigeration load for any given volume of noncondensibles removed from the system.
MANUAL PURGINGSystems that have multiple condenser circuits and that are piped per Figure 1 can be quickly and completely purged of noncondensibles while the system is operating. This is done by closing the inlet valve V1 and waiting 3 to 5 minutes to allow the condenser to fill with liquid from the receiver.
The noncondensibles will flow to the lowest pressure area (the top of the condenser), where it can be purged through valve V2. When condensers are piped per Figure 2, each condenser circuit must be valved off using valves V1 and V4 and then totally vented through V2.
This method is very effective; however, it may not completely purge the coil of all noncondensibles if the manual purge connection is not at the proper location.
Note: This procedure should be performed with caution, by skilled and qualified individuals with proper safety equipment, because a burst of liquid NH3 will be discharged when all noncondensibles are vented.
CONDENSER REFRIGERANT CHARGEWith a trapped outlet, the flow of condensing gas will aid in pushing liquid to the outlet of the condenser, where it can essentially fall into a vertical drain and then be pushed into the receiver by liquid static head pressure. With an untrapped outlet, the gas flow is opposite to the liquid flow at the inlet. This reverse flow tends to push the liquid back into the condenser (Figure 2).
Under certain conditions, such as low ambient temperatures or low loads, the starting of a condenser pump or condenser fan can cause the condenser to “bottle up” or “flood.” This is due to a sudden increase in gas flow caused by a rapid decrease in condenser temperature and pressure. An increased available heat transfer surface is a side benefit of having less liquid charge held in the condenser, but the overall effect is difficult to determine.
PIPINGBy following the pressure drop and flow of Figure 1, it becomes apparent that it is not necessary to purge from the thermosyphon receiver when the liquid drain is trapped. The receiver operates at a pressure higher than the condenser, and there is a constant flow of “pure gas” generated by thermosyphon coolers.
This gas flows through the receiver and will carry any noncondensibles to the condenser, where they will be trapped at the outlet by the liquid seal between the condenser and receiver. Clearly, it is not necessary to purge from a high-pressure receiver if the liquid drain lines are trapped properly.
LIQUID SUBCOOLINGA properly piped condenser that is free of noncondensibles will yield very little liquid subcooling. Liquid that passes through a thermosyphon receiver will lose any subcooling it has gained as it passes through the receiver. Therefore, all compressor capacities should be based on 0 degrees liquid subcooling.
After following the pressure drops, it is apparent that with the simplest system configurations using trapped condenser drains will enhance overall system performance.
Systems that use multiple condensers with multiple condenser circuits create additional piping challenges. These systems require more attention and require Mother Nature’s “liquid static head” to create pressure that will offset unwanted or detrimental pressure drops in the piping.
CONCLUSIONAll systems with thermosyphon oil cooling should use trapped liquid condenser drains.
All systems can take advantage of the benefits of trapping condenser outlets. The benefits of trapped condenser outlets are:
Brown is CEO of Alta Refrigeration, Hampton, GA.
Publication date: 10/07/2002