|
PROCESS/EQUIPMENT HEAT RECOVERY
The recovery
of process heat can significantly reduce fuel/production costs
while optimizing process energy flows. The following recommendations
contained in this module attempt to illustrate the potential 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
- 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.
- Cost of high pressure (125 psig)
steam leaks are on the order of $150 to $500/leak/shift/year
- Cost of low pressure (15 psig)
steam leaks are on the order of $30 to $110/leak/shift/year
- Cost of heat lost through hot,
uninsulated pipes: (associated per 100 feet of uninsulated pipe)
25 psig: $375/100ft/shift/year
50 psig: $430/100ft/shift/year
75 psig: $480/100ft/shift/year
100 psig: $515/100ft/shift/year
- 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:
Thermodynamic properties
of saturated steam (condensed table); (ref. - Energy Management
Handbook, W.C.Turner, editor, pg. 169)
|
Gauge Pressure
(psig) |
Absolute
Pressure (psia) |
Steam Temp
(ºF) |
Enthalpy
of Sat. liquid (Btu/lb) |
Latent Heat
(Btu/lb) |
Enthalpy
of Steam (Btu/lb) |
Specific
Volume
(ft3/lb)
(density)-1 |
|
------------------ |
------------------ |
------------------ |
In a Vacuum |
----------------- |
------------------ |
------------------ |
|
29.74 |
0.0885 |
32 |
0.00 |
1075.8 |
1075.8 |
3306.00 |
|
29.52 |
0.2 |
53.14 |
21.21 |
1063.8 |
1085.0 |
1526.00 |
|
27.89 |
1.0 |
101.74 |
69.70 |
1036.3 |
1106.0 |
333.60 |
|
19.74 |
5.0 |
162.24 |
130.13 |
1001.0 |
1131.1 |
73.52 |
|
9.56 |
10.0 |
193.21 |
161.17 |
982.1 |
1143.3 |
38.42 |
|
7.54 |
11.0 |
197.75 |
165.73 |
979.3 |
1145.0 |
35.14 |
|
5.49 |
12.0 |
201.96 |
169.96 |
976.6 |
1146.6 |
32.40 |
|
3.45 |
13.0 |
205.88 |
173.91 |
974.2 |
1148.1 |
30.06 |
|
1.42 |
14.0 |
209.56 |
177.61 |
971.9 |
1149.5 |
28.04 |
|
-------Psig------ |
------------------- |
------------------ |
------------- |
------------------ |
--------------------- |
------------------ |
|
0.0 |
14.696 |
212.00 |
180.07 |
970.3 |
1150.4 |
26.80 |
|
1.3 |
16.0 |
216.32 |
184.42 |
967.6 |
1152.0 |
24.75 |
|
2.3 |
17.0 |
219.44 |
187.56 |
965.5 |
1153.1 |
23.39 |
|
5.3 |
20.0 |
227.96 |
196.16 |
960.1 |
1156.3 |
20.09 |
|
10.3 |
25.0 |
240.07 |
208.42 |
952.1 |
1160.6 |
16.30 |
|
15.3 |
30.0 |
250.33 |
218.82 |
945.3 |
1164.1 |
13.75 |
|
20.3 |
35.0 |
259.28 |
227.91 |
939.2 |
1167.1 |
11.90 |
|
25.3 |
40.0 |
267.25 |
236.03 |
933.7 |
1169.7 |
10.50 |
|
30.3 |
45.0 |
274.44 |
243.36 |
928.6 |
1172.0 |
9.40 |
|
40.3 |
55.0 |
287.07 |
256.30 |
919.6 |
1175.9 |
7.79 |
|
50.3 |
65.0 |
297.97 |
267.50 |
911.6 |
1179.1 |
6.66 |
|
60.3 |
75.0 |
307.60 |
277.43 |
904.5 |
1181.9 |
5.82 |
|
70.3 |
85.0 |
316.25 |
286.39 |
897.8 |
1184.2 |
5.17 |
|
80.3 |
95.0 |
324.12 |
294.56 |
891.7 |
1186.2 |
4.65 |
|
90.3 |
105.0 |
331.36 |
302.10 |
886.0 |
1188.1 |
4.23 |
|
100.0 |
114.7 |
337.90 |
308.80 |
880.0 |
1188.8 |
3.88 |
|
110.3 |
125.0 |
344.33 |
315.68 |
875.4 |
1191.1 |
3.59 |
|
120.3 |
135.0 |
350.21 |
321.85 |
870.6 |
1192.4 |
3.33 |
|
125.3 |
140.0 |
353.02 |
324.82 |
868.2 |
1193.0 |
3.22 |
|
130.3 |
145.0 |
355.76 |
327.70 |
865.8 |
1193.5 |
3.11 |
|
140.3 |
155.0 |
360.50 |
333.24 |
861.3 |
1194.6 |
2.92 |
|
150.3 |
165.0 |
365.99 |
338.53 |
857.1 |
1195.6 |
2.75 |
|
160.3 |
175.0 |
370.75 |
343.57 |
852.8 |
1196.5 |
2.60 |
|
180.3 |
195.0 |
379.67 |
353.10 |
844.9 |
1198.0 |
2.34 |
|
200.3 |
215.0 |
387.89 |
361.91 |
837.4 |
1199.3 |
2.13 |
|
225.3 |
240.0 |
397.37 |
372.12 |
828.5 |
1200.6 |
1.92 |
|
250.3 |
265.0 |
406.11 |
381.60 |
820.1 |
1201.7 |
1.74 |
|
- |
300.0 |
417.33 |
393.84 |
809.0 |
1202.8 |
1.54 |
|
- |
400.0 |
444.59 |
424.00 |
780.5 |
1204.5 |
1.16 |
|
- |
450.0 |
456.28 |
437.20 |
767.4 |
1204.6 |
1.03 |
|
- |
500.0 |
467.01 |
449.40 |
755.0 |
1204.4 |
0.93 |
|
- |
600.0 |
486.21 |
471.60 |
731.6 |
1203.2 |
0.77 |
|
- |
900.0 |
531.98 |
526.60 |
668.8 |
1195.4 |
0.50 |
|
- |
1200.0 |
567.22 |
571.70 |
611.7 |
1183.4 |
0.36 |
|
- |
1500.0 |
596.23 |
611.60 |
556.3 |
1167.9 |
0.28 |
|
- |
1700.0 |
613.15 |
636.30 |
519.6 |
1155.9 |
0.24 |
|
- |
2000.0 |
635.82 |
671.70 |
463.4 |
1135.1 |
0.19 |
|
- |
2500.0 |
668.13 |
730.60 |
360.5 |
1091.1 |
0.13 |
- Heat air for process
space heating
- Heat water with
process waste heat
- Use exhaust steam
for process heat
- Recover heat from
refrigeration condensers
1. Heat exchangers:
(Heat air for process or space heating)
Use hot process effluents or cooling system discharge fluids to
preheat incoming process fluids. This waste heat can be used
for other process heat, space heat, etc. Recovery of
heat from hot waste water and recycled hot or cold process exhaust
air can be used for process or space heat or cooling, preheating
makeup air, or service or domestic water. Recovery of heat
from air compressors, compressed air dryers, transformers and other
equipment can save energy and be used in the same manner.
The following equation generally illustrates the potential savings
that can be achieved with hot waste air.
Power
Engineering Books
ASHRAE
Thomas Register
CFH = volumetric flow rate of hot waste
fluid , ft3/hr
DA = density of fluid, lb/ft3 (Air
Density Table, water = 62.4)
Cp = specific heat of fluid, BTU/lb-Fº (air = 0.24,
water = 1.0 )
DT = temperature difference for heat exchange, Fº
RF = recovery factor for heat exchanger (from manufacturer)
HY = operating hours per year
h = efficiency of system for which target air is used
Thermo-Tables
BACK
TO FRONT PAGE
2. Heat water with
process waste heat
Processes producing waste heat and requiring preheated water can
benefit from the following recommendation. In general waste
heat from a specific process can be used to preheat water for a
variety of needs through out a manufacturing plant. The following
equation illustrates the potential savings that can be achieved.
Power
Engineering Books
ASHRAE
Thomas Register
GPH = flow rate of water, gal/hr
CF = conversion factor, 8.345 lb/gal
Cp = specific heat of water, 1 BTU/lb-Fº
DT = temperature difference for heat exchange, Fº
RF = recovery factor for heat exchanger
HY = operating hours per year
h = efficiency of system for which target
water is used
Thermo-Tables
BACK
TO FRONT PAGE
3. Use exhaust steam
for process heat
The use of flash steam, steam made from engine exhaust, steam
generated from product condensers, and from condensates in a distillation
process as a heating source can reduce boiler fuel costs.
The following equation illustrates the potential savings that can
be achieved.
Power
Engineering Books
ASHRAE
Thomas Register
PH = pounds per hour of steam/condensate,
lb/hr
EC = energy content of steam at specific temperature and
pressure, BTU/lb (see Thermo-Tables)
HY = operating hours per year, hr/yr
BACK
TO FRONT PAGE
4. Recover heat from
refrigeration condensers.
Heat discharged from refrigeration compressors can be recovered
and used for various applications. Typically this waste heat
is used for preheating makeup water for heating or process use.
The following equations illustrate the potential savings that can
be achieved.
Power
Engineering Books
ASHRAE
Thomas Register
AUH = annual usable heat, BTU
AEU = added electrical use, BTU
TON = tons of refrigeration, Tons
LF = load factor of refrigeration units
COP = coefficient of performance of refrigeration system
CF = conversion factor, tons of refrigeration to BTU/hr,
12,000
HY = operating hours per year
CE = cooling affect, BTU/hr = TON x LF x CF
COP1 = existing system coefficient of performance
COP2 = anticipated system coefficient of performance
Thermo-Tables
BACK
TO FRONT PAGE
|
