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Understanding Oil Return Systems In Unitary Systems

Ed Keuper, a GEA colleague, has presented three (3) very informative postings concerning oil return in refrigeration systems. His emphasis was on chillers and other refrigeration systems that use flooded evaporators and depend on oil separators at compressor discharge as well as a means for returning lubricant from the system to the compressor sump. His key points are (1) that the compressor must always have enough lubricant in the sump and (2) oil must not be present in the heat exchangers, mainly the evaporator, to the extent that heat transfer is significantly degraded.

This sequence deals with direct expansion unitary systems that are designed to move lubricant around the system and back to the compressor sump by momentum, in the case of refrigerant gas transport (Suction and discharge), and solubility where refrigerant is in liquid form. Another way of stating the design objective for this type of refrigeration system is that if, say, 1 % (oil in refrigerant) of lubricant is discharged from the compressor during stable operation, that ratio must be present anywhere in the system at any given time so that the 1 % is safely returned to the sump. One can view this arrangement as two (2) fluid streams – oil and refrigerant – traveling side by side from the compressor, through the condenser and evaporator, and back to the compressor sump after reaching equilibrium.

This series is presented as an overview at best. Many details for piping design to provide oil entrainment and circulation and to prevent oil drainage into places while a system is dormant can be found in the 2010 ASHRAE Handbook of Refrigeration, Chapter 1 – Halocarbon Refrigeration Systems.

Equilibrium and Minimum Run Time

It is important that a system as discussed here – or any system for that matter – be allowed sufficient run time to approach equilibrium after startup. What is equilibrium and how long to reach it? If a compressor is started, transients will occur for a period of time before the lubricant and refrigerant streams settle to a constant, or near constant pace. To be certain that equilibrium is obtained, the very minimum run time after startup can be calculated by dividing the system refrigerant charge by the refrigerant flow rate. For example, if we have a system with refrigerant charge of 20 lb and a flow rate of 900 lb per hr (typical for 5 ton R22 system), the very minimum run time should be 20/900, or 0.022 hours (1.32 minutes). A safe minimum run time in this case might be 2 or 3 minutes. Minimum run time is usually determined by the unitary system manufacturer and programmed into the control system; but, if not, this is one way of calculating it. Run time is usually ensured by a timed on control or some space thermostats. Note that the time required will be system dependent. Most unitary systems apply scroll compressors. Run time information and many other subjects are treated in Copeland Bulletin AE4-1331 R3.

Maintaining an Adequate Lubricant Level

Copeland Bulletin AE4-1331 R3 covers the need to ensure that adequate lubricant always remains in the compressor sump. For instance, they recommend the addition of 1 oz oil for every five (5) lb of refrigerant over 20 lb of refrigerant charge.

A scroll compressor is equipped with about 65 oz lubricant charge. One can quickly calculate the lubricant residing in a system having, say, and 1% lubricant in refrigerant. If the system has 20 lb of refrigerant, the oil in continuous circulation (away from the compressor) would be 20*.01 = 0.2 lb, or 3.2 oz. A system charged with, say, 50 lb refrigerant would have 8 oz lubricant in circulation. This is a substantial portion of the oil charge (12.3%) that cannot be in the sump. Six (6) oz. additional oil, as recommended by the compressor manufacturer, is needed in the system. Note, too, that these calculations are for steady state operation. Many times during transient operation, the circulation rate can be higher, which emphasizes the need for sufficient run time after the compressor is started.

Minimum Velocities in Suction and Discharge Gas Risers

We have to accept the fact that pressure drop of any sort degrades performance. A good rule of thumb for a cooling cycle (looking at the compressor only) is that the compressor COP will drop 2.4% for every degree F drop in evaporating temperature and 1 1/2% for every degree F rise in condensing temperature. Designing tube sizes so that refrigerant velocities are adequate for oil entrainment in suction and discharge risers while minimizing the system efficiency penalties is a delicate balance.

Each low side component (evaporator and suction line) should have a maximum saturation drop of 2F. Each high side component (condenser/receiver and discharge line) should have a maximum of 2 F saturated temperature drop. The liquid line is not as critical as long as you have a liquid feed to the TXV (for subcooling).

Without going through detailed calculations, the industry has successfully applied some rules of thumb for velocities for decades. These rules are to use minimum refrigerant velocities of 1000 ft/minute for vertical vapor risers and 750 ft/minute for horizontal vapor lines. This requirement also applies to individual evaporator and condenser circuits that require the transport of gas vertically.

Vapor lines can pretty easily meet these criteria for single capacity systems. However, if a system is designed to provide more than one level of capacity, the designer must be certain that the required velocities are operative at the system’s lowest capacity. This means, of course, that the system at full capacity might have to have velocities considerably higher than 1000 ft/minute. If a designer wants to be more precise so that multiple capacity systems can operate as efficiently as possible, refer to 2010 ASHRAE Handbook of Refrigeration, Chapter 1 – Halocarbon Refrigeration Systems.

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