Heat and power around the big fan units

This is a record of various measurement taken on various components of the air cooling system.

(I)

Air is taken in at room temperature. The heat exchangers are off, and the outlet temperature is measured at T, while the flow is measured at A.


ps version
         inlet temperature:   70 deg F
         outlet temperature  107 deg F    -->  temp rise = 37 F
         flow velocity        36.7 m/s
         tube diameter           2"       -->  volume flow = 74 l/s
         1 l air weighs 1.2 g             -->  mass flow   = 89 g/s 
         1 lb  = 0.45 kg                                   = 0.2 lb/s

         specific heat of air  0.24 btu/lb F
         1 btu = 0.00029kWh

 it takes 0.24 btu to heat 1 lb air by 1 F
          0.07 Wh          1           1
          0.52 Wh          0.2         37 F
          1865 Ws          0.2         37 F
so it takes 1865 Watts to heat the air in the top branch in 1 second. The flow at B was 33.3 m/s, so the total heat added is about 3560 W. [The anemometer opening diameter is actually 2 7/32", and if you scale by that, the power estimate drops to 2900 W. Clearly there are 15% uncertainties here.]

Empirically, power(W) = 1.12 × DeltaT(°F) × v(m/s)

(II)
For the next set of measurements, the long hoses were removed from the heat exchanger box. A short (30 cm) piece was used on one exit to measure the exit temperature T2.

ps version

IIa) With both chillers off:

room temperature =
temperature T1 =
T2 =

69.5°F
101.5°F
101.5°F

Using the flows measured below (52.4 and 66 m/sec), we have 1878+2365=4243W added to the airstream.

IIb) Turn on the air cooling chiller, setpoint 0°C, stabilizes at 23.6°C running continuously.

Flow at T2, handheld at the end of the hose:
at the unconnected end:
T2:

52.4 m/s
66.0 m/s.
91°F

These streams have 1261+1589=2850W of added heat, so that about 1400W was taken out by the cooling chiller/heat exchanger.

IIc) Now also turn on the dehumidifier chiller. Setpoint setpoint 0°C, stabilizes at 13.8°C running continuously.

T2 = 89.6°F

Total estimated power removed by the dehumidifier chiller/heat exchanger is only about 185W.

IId) Plug up the unconnected end. Air velocity at T2 now about 95 m/s, at which point the anemometer bearings melted.

(III)
The manufacturer suggested that simply throttling the flow with a vane ($300) would reduce power consumption. This seemed a bit counterintuitive, so I did the following measurements (the hardware is arranged as in a).):

IIIa) The air intake port on the filter housing is a 2 7/8" i.d opening, and I used different blocks of wood to partially block this opening.


ps version

d T (°F) v (m/s) P (in.water) currents (A)
- 104.7 30.4 8.5      
1 1/8" ? 25.5 16      
1 1/2" 107 21.6 off scale 10.5 9.8 10.6
1 7/8 112 15.5 off scale 9.0 8.5 9.1

Note that as the flow goes down, the temperature rises. However since the power used by the motor goes down as the flow is restricted, the heat in the airstream must also go down.

IIIb) The manufacturer actually specified that the vane should be installed after the turbine, not before as under IIIa, so I built a blade valve to go between the turbine and the heat exchanger box.


ps version
blockage* filter pressure
(in.water) 		v
(m/s) 		currents
(A)
0% 9.3 25-26 12.8 11.7 12.9
20% 9.2 26-27 12.9 11.9 13.0
40% 7.4 23-24 12.0 11.1 12.1
60% 4.3 17-18 10.3 9.7 10.6
70% 2.3 11-12 9.0 8.4 9.2
80% 1.1 7 7.6 6.9 7.5

*the blockage is the distance b as a percentage of the hole diameter. Below are plotted the filter underpressure, air flow velocity at the end of the 2" hose, and the currents in the 3 phases to the motor.


ps version

the last plot is the mean current vs. the flow velocity. It shows that if you cut the flow in half, you reduce the current, and therefore the added heat, by 36%.
Last update 3 Feb 99 HvH
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