As the temperature of intake air increases, the air density decreases, thus the mass flow and pressure capability decrease. To make up for this reduction, air compressor start consuming more power and hence redirecting the outside cold air for compressor air intake will result in less consumption of energy

1. Redirect outside airfor compressor air intake
Existing Practice and Observation
The air supplied to the compressor’s intake is drawn from the ambient air that is inside the facility
where the compressors are located. Due to the high temperature of the compressors’ exhaust, the
compressors are drawing in air that is at a higher temperature. On average, the temperature inside
the facility is higher than that of outside air during the colder months of the year while it’s the
other way around in the warmer months, when the outside air is warmer than the inside air. As the
temperature of intake air increases, the air density decreases, thus the mass flow and pressure
capability decrease. (Source: US Department of Energy Compressed Air Tip Sheet). To make up
for this reduction in capacity, the air compressor must consume more energy.
Recommended Action
To make up for this temperature increase in the air intake, it is recommended that a ducting or a
pipe line be installed to connect the intake on the compressor to the outside through either the wall
near the compressor, or the roof. This can be implemented to get air from the outside when it is
colder than the air that is inside the facility. The estimated time that this intake should be utilized
would be during the months of September until May. To keep the quality of air that the
compressors will intake, it is important to also include a filtration system in order to keep the pipe
or duct work free from contaminates.
Analysis
The average temperature outside the facility during May is 66 ˚F (18.8 ˚C) [the measured
temperature outside of the facility on 6th May was 57.2 ˚F (14 ˚C)], while the area around the
compressors where the air is entering the compressor was measured to be 75.2˚F (24 ˚C)
approximately.
The savings associated with utilizing lower temperatures for the compressor intake depend on
the power draw at higher temperature verses that of a lower temperature. If a polytropic
expansion is assumed of the air intake, the relationship is one of absolute temperature of the air.
The fractional savings can then be determined as:
FS = (Thi – Tlow) / Thi
Where,
Thigh = Temperature at high (K)
Tlow = Temperature at low (K)

2. Therefore, the fractional savings can be determined as:
For the month of May, for example, using the average temperature from the Table shown below
FS = [(24+273) - (18.8+273)]/ (24+273)
= 0.02 %
The average temperatures (outside air) for each of previous months along with their fractional
savings are tabulated below:
S. No Month
Average
Temperature ˚F
(˚C)
FS
1 January 30˚F (-1.1˚C) 0.08%
2 February 34˚F (1.1˚C) 0.08%
3. March 48˚F (8.8˚C) 0.05%
4. April 55˚F (12.7˚C) 0.04%
5 May 66˚F (18.8˚C) 0.02%
6 June 75˚F (23.8˚C) 0%
7 July 79˚F (26.1˚C) -0.01%
8 August 80˚F (26.6˚C) -0.01%
9 September 72˚F (22.2˚C) 0.01%
10 October 58˚F (14.4˚C) 0.03%
11 November 47˚F (8.3˚C) 0.05%
12 December 35˚F (-0.5˚C) 0.08%
Note: For the months of June, July & August there are no savings for drawing outside air. So,
inside air is better during those months of the year.

3. There are four 100 HP compressors, two run 80% of the plant operating hours (0.8 x 8,760 hrs =
7,008 hrs i.e. 584 hrs monthly) while other two runs 40% of plant operating hours (0.4 x 8,760
hrs = 3,504 hrs i.e. 292 hrs monthly). Based on this, monthly savings can be calculated:
Sample Calculation:
For the month of May, the fractional savings are calculated as 0.02 %
The average energy savings, known as AES, can be determined from:
AES1 =
𝐻𝑃 𝑋 𝐻𝑌 𝑋 𝐹𝑆 𝑋 𝐿𝐹1 𝑋 0.7465
η
x N
AES2 =
𝐻𝑃 𝑋 𝐻𝑌 𝑋 𝐹𝑆 𝑋 𝐿𝐹2 𝑋 0.7465
η
x N
AES = AES1 + AES2
Where,
HP = Nameplate horsepower = 100 HP
HY = Hours of operation = 730 hours (max. operating hours = 8,760/12 = 730 hrs monthly)
LF1 = Load factor = 0.80
LF2 = Load factor = 0.40
N = Number of compressors = 2
 = Efficiency of the compressor = 0.90
Therefore, the average energy savings calculated would be:
𝐴𝐸𝑆 =
100 ×730 × 0.02 ×0.80 ×0.7465
0.90
x 2 +
100 × 730 × 0.02 ×0.40 ×0.7465
0.90
x 2
𝐴𝐸𝑆 = 1,937.58 + 968.79 = 2,906.37 kWh
Energy & Cost Savings
For the month of May, total energy cost savings, known as ECS,
ECS = AES x Ce

4. Where,
Ce = Electricity cost per kWh
Therefore,
𝐸𝐶𝑆 = 2,906.37 𝑘𝑊ℎ × $0.0474 = $137.76 annually
Note: For the months of June, July & August there are no savings for drawing outside air. So,
inside air is better during those months of the year.
Month AES (kWh) ECS ($)
January 11,625.48 551.04
February 11,625.48 551.04
March 7,265.92 344.40
April 5,812.74 275.52
May 2,906.37 137.76
September 1,453.18 68.88
October 4,359.55 206.64
November 7,265.92 344.40
December 11,625.48 551.04
Total
63,940.12
3,030.72

5. Emissions Reduction Analysis
For the total calculated usage savings of 63,940.12 kWh, the annual reduction of carbon dioxide
is:
= 63,940.12 kWh x 1.84
= 117,649.82 pounds (pounds of CO2 per kWh = 1.84)
-Source of emission coefficients is the Energy Information Administration.
Implementation Cost and Payback
The cost of installation would include the cost of aluminum ducts for outside air intake and would
include labor cost. With an estimation of 80 hours of labor, the labor cost is $40 per hour, so the
total labor cost is $3,200. Duct material costs approximately $1,500. The total cost of the
installation is $4,700
At this project cost the payback period would equal:
$4,700/ $3,030.72= 1.55 Years
References:
1. https://www.ncdc.noaa.gov/