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PROCESS HEAT CONFINEMENT


       The confinement of process waste heat can reduce heating fuel costs, aid in process efficiency, and optimize process energy flows.   The following module containing the recommendations below illustrate the savings that can be achieved.

General Rules of Thumb:

  • The average cost of electricity is $0.05/kWh ($15/MMBtu)
  • The average cost of natural gas $0.35/CCF
  • The average cost of #2 fuel oil is $4/MMBtu
  • There are 2000 hours per year per shift (based on the assumption that one shift is 8 hours per day, 5 days per week, 50 weeks per year)
  • A typical boiler or furnace has a combustion efficiency of 80%
  • 90% of the heat loss from a hot, uninsulated surface can be economically eliminated by installing insulation.
  • Switching from electric heat to natural gas or #2 fuel oil can reduce heating costs by 78%
     Notes:
                    Before choosing the following targeted recommendations READ THE FOLLOWING:     Pay back estimates for the following recommendations will use the equation below.  They will vary depending on the, application, type of installation, and purchase quantity of material and labor associated with each recommendation.  It will be up to the person doing the analyses to use the URL references below each equation to help estimate an implementation cost.

    The data correlating to the variables below each equation will be prompted for in order to execute a calculation.  Frequently the fuel cost (FC) associated with the specific recommendation will be prompted for in order to calculate the annual cost savings (ACS). Unless otherwise specific to a particular recommendation the ACS will be calculated as follows:
 
    1. Insulation repair or replacement
    2. Cover open tanks with floating insulation
    3. Seal open tanks
    4. Eliminate or reduce openings
    5. Minimize ventilation use
    6. Install variable speed drives/reduce motor HP
    7. Use outside air for process cooling (short cycle exhaust air)

1.  Insulation repair or replacement
    Exposed surfaces radiating heat can contribute to energy losses that are not always obvious.  Inspection of surfaces of a temperature above 150 ºF should be conducted to ensure that they are adequately insulated.  Install, increase or repair insulation on process tanks, vessels, lines, and equipment.
    One of the most common reduction in insulation performance is moisture.  Most insulating materials rely on air spaces for effective insulation properties, therefore  anything that reduces the size of or fills those air spaces will tend to reduce the thermal resistance.  The following equation and link will illustrate the potential savings that  can be achieved.
 
Insulate surfaces


Heat loss from a flat surface (Btu/hr-ft2)*
 
Surface
-----------------
----Temperature
Difference (Fº)
-----------------
-----------------
------------------
Type:
50
100
150
200
250
300
FLAT
98
215
360
533
738
978

*(ref. - Energy Management Handbook, W.C.Turner, editor, pg. 489)

Pipe insulation resource (University of Massachusetts)
Data Conversions?
RPN Calculator?
 

2.  Cover open tanks with floating insulation.
        Heated process tanks used for anodizing, plating and cleaning operations suffer costly heat loss through the exposed liquid surface.  This increased heating load can be reduced by insulating the exposed surface with floating insulation.  The following equation and tables of data provide an insight the the potential energy savings.


*(40% relative humidity)
Water Temp ºF*
Heat loss W/ft2
80
40
100
80
120
155
140
270
160
445
180
700
200
1075

 
    CHL = current heat loss from exposed tanks, W/ft2
    A = exposed surface area, ft2
    F = percent reduction in energy use (see table below)
    HY = operating hours per year
     h = efficiency of tank heating system

        Data obtained below from tests conducted at the Technological Institute of Copenhagen.

Temperature ºF
Temperature ºC
No. of balls
   Heat loss, KW/hr 
(1 layer)
Percent reduction
   Heat loss       (2 layers)
Percent reduction
122
50
1.7
0.6
65
0.5
71
158
70
4.6
1.3
72
0.8
83
194
90
10.7
2.7
75
2
81
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3.  Cover and seal open tanks.
        Heated process tanks used for anodizing, plating and cleaning operations suffer costly heat loss through the exposed liquid surface.  This increased heating load can be reduced by insulating the exposed surface with floating insulation.  The following equation provides an insight the the potential energy savings that can be achieved.
    Power Engineering Books
    ASHRAE
    Thomas Register

    RA = rate of evaporation, determined from table below, given the air velocity between air and water, and the average water temperature, lb/ft2-hr (ref.- Handbook of Data Sheets for Solution of Mechanical Systems Problems, R.W. Roose, P.E.)
     

     
    ------------------------
    ------Temp. (ºF) of
    exposed water---
    ------------------------
    -------------------------
     
    100
    120
    150
    180
    200
    Air Velocity 
    (fpm)
    --------------------
    Water Evaporated
    (lb/ft2-hr)
    --------------------
    --------------------
    100
    0.17
    0.38
    0.95
    2.1
    3.35
    80
    0.16
    0.355
    0.9
    2.0
    3.15
    70
    0.15
    0.33
    0.85
    1.59
    3.0
    60
    0.14
    0.325
    0.825
    1.58
    2.9
    50
    0.135
    0.3125
    0.8
    1.575
    2.75

    A = exposed surface area, ft2
    LE = latent heat of vaporization, BTU/lb,  Thermo-Tables
    HY = annual tank operating hours
     h = efficiency of tank heating system

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4.  Eliminate or reduce openings
         Openings that are not designed for process heating and/or building heating use, can increase plant operating costs.  The elimination of these openings will reduce the unnecessary heat loss from the above systems.  Reduce size of charging openings, slots, doors, etc., or add a movable cover or door on process equipment.  This will partially reduce radiation and convection losses.  To make calculations easier, it may be possible to assume that radiation losses are negligible. The following equation can provide an insight to the potential savings that can be achieved.
     

    Power Engineering Books
    ASHRAE
    Thomas Register
     

    SIG = sigma, Steffan-Boltzmann constant, 0.1713 x 10-8 BTU/hr-ft2-R4
    A = area to be covered, ft2
    TO = temperature of opening, R (degrees Rankine = degrees ºF + 460)
    TR = temperature of room, R (degrees Rankine = degrees ºF + 460)
    EPS = epsilon, emissivity of opening, (estimated)
    HY = operating hours per year
    h  = efficiency of heating system

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5.  Minimize ventilation.
        Excess ventilation due to the neglect of a heating or cooling system can be a energy sink.  Depending on the air quality issue ventilation systems should be operated at design conditions.  Reduce ventilation air to a safe minimum level; reduce building exhausts and make-up air; close outdoor air dampers during warm-up or cool-down periods.  Use minimum necessary ventilation to drive off combustible solvents or other unwanted vapors; revise smoke cleanup from production operations.  Switch off exhaust fans when not needed.  Energy savings will come from a reduction in electricity used (fan energy savings, FES) and a reduction in the heating load (heating energy savings, AES).
6.  Install variable frequency drives, or reduce motor horsepower.
        A variable or adjustable speed drive (ASD or VSD) will reduce the speed of a motor by adjusting the frequency, voltage, or current of the motor input so that the motor performance just matches the present load.  The speed of AC motors is proportional to the frequency of the power supply.  AC adjustable drives convert 3 phase 60 Hz to an adjustable frequency and voltage source for controlling the speed of AC squirrel cage induction motors.  Moreover, VSD's or ASD's control motor speed by synthesizing the voltage and frequency of power supplied to the motor so that it runs only as fast as necessary to do the work required at a given moment.  VSD's or ASD's can control speed over a wide range, from 0 to 300% of the rated speed.  There are four basic types: 1. Inverter-based, 2.  Cycloinverters, 3.  Wound-rotor slip recovery, and 4.  Voltage-level controls.  ASD's and VSD's can provide accurate process control, and match the speed of  a motor-driven device to varying load requirements.
        Motor horsepower reduction will not necessarily match the present load, however it can reduce fan energy consumption especially when the current motor(s) are under or over sized  There will be heating energy savings (HES) and fan energy savings(FES) that can be derived in this recommendation.  The following equation illustrates the potential savings that can be achieved

    Power Engineering Books
    ASHRAE
    Thomas Register
     

    CCFH = current air flow rate, ft3/hr
    PCFH = proposed air flow rate,  ft3/hr (after VSD implementation or horsepower reduction)
    Cp = specific heat of air, 0.24 BTU/lb-Fº
    DA = density of air, lb/ft3 Air Density Table
    Ta = ambient space temperature, ºF
    Tw = outside temperature, ºF (average winter temp., obtained from U.S.A.F. Bin Weather Data)
    TMY weather files
    HY = hours per year during heating season
    h  = heating system efficiency

    Data Conversions?
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    Press the calculate button to execute an estimation

    Power Engineering Books
    ASHRAE
    Thomas Register

    HP = fan horsepower
    CON = conversion factor, 0.7465 KW/hp
    LF = load factor of motor(s)
    PS = fractional power savings for reduced air flow
    HY = hours per year during heating season
    h  = heating system efficiency
     

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    Press the calculate button to execute an estimation

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7.   Use outside air for process cooling (Short cycle exhaust air).

         Process cooling systems that can utilize outdoor air as a medium under conditions where there is as favorable temperature difference.  This reduces the demand for energy and thereby reducing cooling operating costs.  Use outside air instead of conditioned air for process purposes.  In process drying for example, use dryer combustion air, etc.  Use an independent air source to reduce the proportion of conditioned air removed by exhaust hoods, smoke cleanup, or other process exhaust.  The following equation illustrates the potential savings that can be obtained.
 
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