Net Oil Pressure Vs. Discharge Pressure

January 5, 2001
/ Print / Reprints /
ShareMore
/ Text Size+
Many larger compressors in the refrigeration and air conditioning field have forced oiling systems. These compressors are usually over 5 hp. They contain an oil pump located at the end of the compressor’s crankshaft (Figure 1).

The crankshaft is actually connected to the oil pump and supplies power, which turns the oil pump.

Oil pumps can be of the gear or eccentric type (Figure 2). The oil pumps force oil through drilled holes in the crankshaft and deliver it to bearings and connecting rods. The oil then drops to the crankcase to be picked up again by the oil pump.

Smaller compressors usually have some type of splash type oiling system. These systems have an oil scoop that scoops and flings the oil throughout the crankcase, causing an oil fog as the crankshaft turns.

When dealing with compressors that employ an oil pump, many service technicians confuse net oil pressure with oil pump discharge pressure. However, it is of utmost importance that service techs understand the difference between these two pressures when servicing compressors with oil pumps.

The oil pump’s rotating gear or eccentric adds a certain pressure to the oil pumped through the crankshaft. This pressure is considered “net” oil pressure. Net oil pressure is not the pressure that can be measured at the discharge of the oil pump. The oil pump picks up oil (at crankcase pressure) from the compressor’s crankcase through a screen or filter (Figure 3).

The oil pump discharge port’s pressure includes both crankcase pressure and oil pump gear pressure it adds to the oil. This is why net oil pressure cannot be measured directly with a gage. A gage at the oil pump’s discharge port would register a combination of crankcase pressure and oil pump gear pressure.

The technician must realize this and subtract the crankcase pressure from the oil pump discharge port’s pressure to get the net oil pressure. Equation 1 shows how to do this:

(Oil pump discharge pressure) – (Crankcase pressure) = (Net oil pressure)

For example: The oil pump discharge pressure is 80 psig. The crankcase pressure is 20 psig. What would be the net oil pressure?

Solution: Subtract the crank-case pressure from the oil pump discharge pressure to get the net oil pressure; that is, (80 psi) - (20 psi) = 60 psi net oil pressure. This means the oil pump is actually putting 60 psi of pressure into the oil when delivering it into the crankshaft’s drilled passages.

This is why oil safety controllers are called differential-type controllers. They sense the difference between oil pump discharge pressure and crankcase pressure. This is why these controllers have a capillary tube or pressure transducer connected to the discharge of the oil pump and the crankcase to sense a difference of pressures or “net oil pressure” (Figures 4 and 5).

(Author’s note: Oil safety controller types and their circuitry will be covered in next month’s column.)

Net oil pressures vary from compressor to compressor, usually ranging from 20 to 40 psi. Most oil pressure safety controllers will shut the compressor down if the net oil pressure falls below 10 psi.

Variables that effect the net oil pressure include:

  • Compressor size;
  • Viscosity of the oil;
  • Temperature of the oil; and
  • Bearing clearance.
  • Larger compressors need more net oil pressure because they have more surface areas to lubricate. The oil pumps must also pump and carry the oil greater distances within the larger compressor.

    Also, as the oil gets hotter and its viscosity drops, the net oil pressure also usually drops. As a compressor wears, its tolerances become greater and easier for the oil to escape through its clearances.



    VACUUM MEASUREMENTS

    Sometimes the compressor’s crankcase may be operating in a vacuum. In this case, the crankcase pressure is negative. (Remember that every 2 in. of mercury vacuum is equivalent to 1 psi.) For example: What is the net oil pressure if the oil pump discharge pressure is 35 psig and the crankcase pressure is 6 in. of vacuum (-3 psi)? Solution: Again, using Equation 1, we must subtract the crankcase pressure from the oil pump discharge pressure to get the net oil pressure. (Oil pump discharge pressure) - (Crankcase pressure) = (Net oil pressure) (35 psi) - (-3 psi) = 38 psi of net oil pressure. This means that the oil pump is delivering 38 psi of net oil pressure through the crankshaft and bearings.

    Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI.

    Publication date: 01.08/2001

    Did you enjoy this article? Click here to subscribe to The NEWS Magazine

    You must login or register in order to post a comment.

    Multimedia

    Videos

    Image Galleries

    2014 MCAA Annual Convention

    Scenes from the 2014 MCAA Annual Convention in Scottsdale, Ariz.

    Podcasts

    NEWSmakers: Julian Scadden

    Training is an ongoing process. Julian will discuss how you can generate maximum return on time and energy invested training by following a three part process. Listen to this podcast to get expert tips on training, tracking and follow up. 

    More Podcasts

    ACHRNEWS

    NEWS 04-21-14 cover

    2014 April 21

    Check out the weekly edition of The NEWS today!

    Table Of Contents Subscribe

    SERVICE CALLS POLL

    Which statement on service calls best applies to your business?
    View Results Poll Archive

    HVACR INDUSTRY STORE

    plumbing-hvac.gif
    2014 National Plumbing & HVAC Estimator

    Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

    More Products

    Clear Seas Research

     

    Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

    DON'T MISS A THING

    Magazine image
     
    Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

    STAY CONNECTED

    facebook icontwitter iconyoutube iconLinkedIn i con