Basic AHU Operation Question

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Looking to get a basic understanding of AHU operation. For an AHU with two compressors and two condenser fans, can both compressors and both fans operate simultaneously? or are they interlocked? What is the purpose of having two compressors? Redundancy, Efficiency, etc.? Thanks in advance.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
They can operate independently or simultaneously.

In the past, the main reason for two compressors was the ability to more-closely match the unit's capacity to the cooling load of the moment without using variable-frequency drives. Sometimes the compressors were two different sizes, enabling three different capacities.

Better load matching offers better humidity control, improves longevity by reducing the number of on-off cycles, and may reduce the cost of demand charges. I don't think it improves energy efficiency.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
One stage, two stage and some have three stages. If the first stage isn't enough to satisfy, #2 kicks in. If that's not enough #3 kicks in. Controlled by the thermostat on the wall. So yeah, they all can run at the same time. The total load (all compressors running) is whats reflected on the nameplate.

Edit: many times the evaporator blower is on a VFD, not the compressors (in commercial applications).

-Hal
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
... many times the evaporator blower is on a VFD ...
Yes. Changing the fan speed changes the ratio of cooling-to-dehumidification, and a VFD enables the machine to simultaneously match both the cooling load and the humidity load of the moment, and adapt to either a clear sunny day or a cool rainy day.

The motor driving the evaporator blower (on the indoor, cold coil) is much, much, much smaller than the motor driving the compressor.

"Evaporator" refers to the refrigerant evaporating on the inside of the coil, not the atmospheric moisture condensing on the outside of it, which is also happening ... my apologies for the Fundamental Principle of Eternal Maximum Confusion once again popping up.
 
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