AR No. __ - Install Occupancy Sensors
Estimated Electric Energy Savings = 108,928 kWh/yr; 371.8 MMBtu/yr
Estimated Electric Energy Cost Savings = $2,590/yr
Estimated Electric Demand Savings = 161.1 kW/yr
Estimated Electric Demand Cost Savings = $1,220/yr
Estimated Total Cost Savings = $3,810/yr
Estimated Implementation Cost = $27,530
Simple Payback = 7.2 years
Recommended Action
Circulating pumps should be installed to provide agitation for the galvanizing pre-treatment line.
By eliminating the current practice of compressing air and then expanding it back to a lower
pressure, circulating the tank fluid can provide equivalent agitation with reduced electrical
consumption.
Background
Chemical treatment tanks often require agitation to prevent settling and thermal stratification.
Traditionally, fluid motion has been induced by mechanical stirrers or bubbling air through the
solution. An alternative method is to circulate the solution with a pump. The pumped fluid is
forced through a specially designed nozzle at high velocity, inducing additional flow through the
nozzle exhaust. Thus, pumping one gallon creates five gallons of fluid flow, effectively increasing
the agitation.
Using the tank fluid for agitation provides several benefits over the other methods of agitation.
1. Pumping requires less energy than either mechanical or pneumatic agitation.
2. A smoother fluid surface is maintained, thus reducing surface heat losses and
evaporation.
3. Misting and fumes caused by air agitation are eliminated.
4. Contamination of the solution from oil and particulates in the compressed air is
eliminated.
A 150 hp compressor supplies air to the galvanizing building. A significant amount of the air
supplied by the compressor is used to agitate the caustic clean, caustic rinse and pickling tanks. The
compressor operating pressure during the site visit was 125 psig (138.8 psia), but the pressure at the
pre-galvanizing line is estimated to be 110 psig.
Anticipated Savings
Energy savings, ES, and energy cost savings, ECS, due to the use of pumped fluid agitation rather
than compressed air are estimated as follows:
ES  PC  PP   H  C 1
ECS  ES  avoided cost of electricity
where
PC =
power consumed by compressor to agitate tank, hp
PP =
H =
C1 =
power consumed by the pump to agitate tank, hp
operating hours, h/yr
conversion factor, 0.746 kW/hp
The power required to compress the volume of free air Vf needed for agitation from atmospheric
pressure to the compressor discharge pressure can be calculated as follows1:
k 1


k
 Po   N
 k 



Pi  C 2  V f  
 1
  N  C 3   

P
 k  1
 i 

PC 
E ac  E mc
where
Pi
C2
Vf
k
N
C3
Po
Eac
Emc
=
=
=
=
=
=
=
=
=
inlet (atmospheric) pressure, psia
conversion constant, 144 in2/ft2
volumetric flow of free air, cfm
specific heat ratio of air, 1.4 (no units)
number of stages, no units
conversion constant, 3.03 x 10-5 hpmin/ftlb
pressure at compressor outlet, psia
air compressor isentropic (adiabatic) efficiency, 0.82 no units
compressor motor efficiency, no units
The compressor's rated capacity is approximately 665 cfm. During the site visit, two 3/4" and four
1/2" diameter hoses were used for the agitation of the three tanks. Internal diameters are estimated
to be 3/8" and 1/4". Calculations based on a line pressure of 110 psig and the hose diameters
estimated a total air flow of 625 cfm. It seems unlikely that tank agitation uses 94% of the
compressor capacity. This analysis assumes tank agitation consumes 300 cfm or approximately
45% of the compressor capacity. The compressor motor efficiency is 92.8%. Thus, the power that
is currently consumed by the compressor to provide air for tank agitation is estimated as follows:

PC 
13.8 144 300  1.4 1 3.03  10 5
 0.4 
0.82 0.928 
0.4


1.4
138.8





1

 13.8 


  78 hp

Plant personnel can determine the actual air usage by measuring the compressor cycle time when no
other pneumatic equipment is operating. During the site visit, measurement of the compressor
motor speed indicated a motor power draw of about 102 hp. Thus, this analysis assumes tank
agitation accounts for about 76% of the compressor power draw.
1
Chapters 10 and 11, Compressed Air and Gas Handbook, Fifth Edition, Compressed Air and Gas Institute,
New Jersey, 1989.
The power consumption of the proposed pumping system depends upon the flow requirements, and
pump sizes. The pumping flow rate, Q, required for effective agitation is calculated from the
following equation:2
PF 
where
TV =
TO =
C4 =
TV  TO  C 4
5
tank volume, ft3
turnover, the number of times the tank volume will be circulated per hour, 1/h
conversion constant, 0.125 gpm/cu. ft per hour
Assuming that ten turnovers could provide adequate agitation for the cleaning tank, the pumped
flow for the caustic cleaning tank can be calculated as follows:
PF 
5.5  5  48 10 0.125   330 gals/m in
5
The dimensions, turnover rate and required flow rate for all the tanks considered are shown in the
table below.
Pumping Flow Rates Required for Agitation
Pump
Depth Width Length Turnover Flow
Tank
ft
ft
ft
gpm
Caustic
5.5
5
48
10 330
Caustic Rinse
5.5
5
48
10 330
Pickling
6.5
7
48
10 546
The required flows could be provided with two 7.5 hp motors for each of the caustic clean and
caustic rinse tanks, and two 15 hp motors for the pickling tank, for a total rated power of 60 hp. The
galvanizing line operates 8,112 h/yr. Assuming the pump motors operate at their rated capacity, the
energy savings that can be realized by using pumped fluid instead of compressed air to provide
agitation is estimated as follows:
ES  78  60 8,112 0.746   108,928 kWh/yr
ECS  108,928 kWh/yr $0.0238/kWh   $2,590/yr
The demand savings, DS, and demand cost savings, DCS, can be estimated as follows:
2
From Serfilco '94-'95 Catalog "U" pp. 132 - 134
DS  PC  PP  C 1  C 5  CF  DUF
DCS  DS  avoided cost of electric demand
where
C5 =
CF =
DUF =
conversion constant, 12 months/yr
fraction of rated power contributing to the facility peak demand, per month
fraction of the year equipment contributes to peak demand, no units
The tanks require agitation during all working hours, so CF = 1.0/month. The plant operates all
year, so DUF = 1.0. Thus,
DS  78 - 60 0.746 12 1.0 1.0   161.1 kW/yr
DCS  161.1 kW/yr $7.55/kW   $1,220/yr
Thus, the total estimated electrical cost savings are $3,810/yr. Additional energy savings will result
from reduced heating requirements for the tanks.
Implementation Cost
Implementation of this AR involves purchase and installation of circulating pumps, piping and the
eductor nozzles. The estimated implementation cost is shown in the table below.
Unit
Cost
Total
Cost
Tank
System
Caustic Clean
12 Eductor systems: 4 ft. long CPVC w/ 4 x 3/4" eductors
$157.95 $1,895
2 Pump/motors (7.5 hp each), rated at 215 gpm @ 50 ft. $3,857.21 $7,714
TDH
Caustic Rinse
12 Eductor systems: 4 ft. long CPVC w/ 4 x 3/4" eductors
$157.95 $1,895
2 Pump/motors (7.5 hp each), rated at 215 gpm @ 50 ft. $3,857.21 $7,714
TDH
Pickling
16 Eductor systems: 3 ft. long CPVC w/ 3 x 3/4" eductors
$127.04 $2,033
2 Pump/motors (15 hp each), rated at 350 gpm @ 75 ft. $3,140.42 $6,281
TDH
TOTAL
$27,532
The total implementation cost is $27,530. Thus, the cost savings of $3,810/yr will pay for the
implementation cost in about 7.2 years.
Contact Information
Vendor contact name:
Vendor company name:
Address:
City:
State:
Phone:
Fax:
E-mail:
Internet:
Make and Model Number:
Other Info: