Fan Cycling will Increase Condenser Head Pressure. The Sarah Connor Control Chronicles

Fan Cycling?  No, I’m not a fan of cycling.  Oh, I know!  It’s about an exercise bike using an air-resistant fan.  Is that “Fan Cycling?”


Eh, Sarah Connor here, with another edition of the Sarah Connor Control Chronicles, where we talk about products that you know and love.  Today, we are talking about Fan Cycling, (which by the way has nothing to do with bikes).  It’s all about pressure, and how you have to switch fans off and on to regulate the condenser head pressure.  At first glance, the whole concept of condenser head pressure reminded me of high school physics, and that reminded me of how much I really never understood thermodynamics.  But being the controlling person that I am, here goes a very cursory explanation of how fan cycling is used to control condenser head pressure.


When the ambient temperature drops, the condenser head pressure drops.  Normally, that would be a good thing, because you know how I love to save energy.  With a greater temperature difference and lower condenser head pressure, the condenser capacity increases.  Energy consumption is reduced.  But too much of a good thing is really a bad thing.  When condenser head pressure drops too much, performance suffers, and the whole system can be damaged.  When the underfed thermostatic expansion valves (TEV) starve the evaporators, there is an increase in discharged air temperature.  Superheating is bad.  Also, oil logging occurs. This happens because the mass flow of the refrigerant slows, so evaporation will decrease.  If the velocity is too low, the oil can’t return up the suction riser, so it logs in the evaporator.  The compressor oil failure controls may even trip.  Then after a defrost cycle, a flood of oil returns, possibly damaging the compressor.  If you underfeed the TEV, even though the BTU load will be reduced, it will increase the suction gas temperature.  When this goes up, the discharge temperature goes up proportionately.  This means inefficiency, and I, Sarah Connor, am all about defeating energy inefficiency.


So how does this apply to fan cycling?  One of the most common ways to regulate condenser head pressure is by turning fans off and on triggered by pre-set pressure ranges.  Johnson Controls offers a wide range fan cycling products.  The P70AA-118C offers single and dual pressure control for non-corrosive refrigerants such as R-12, R22, R-500, or R-502.  This can handle pressure ranges from 100-400 psig.  Johnson Controls also offers the P170AA-118C and SEC Ultra Cap, the P470 Electronic Pressure Control and Transducer.  Choose the System 450 for multiple motors.  When you need variable speed control, there is the P266 and VFD66.


When it comes to fan cycling, look to Stromquist to supply these and all Johnson Controls products.  Call upon the Stromquist support experts who are always here to help, just in case you need a little assistance in selecting the right fan cycling product.


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