AR No. _ - Insulate ______Tanks
Estimated Gas Energy Savings = **.* MMBtu/yr
Estimated Gas Cost Savings = $***/yr
Implementation Cost = $***
Simple Payback = *.* years
Recommended Action
The _____ tanks should be insulated to reduce heat losses from the tank surface.
Background
The _____ tanks are not insulated, resulting in high surface temperatures. By adding insulation, heat loss will be minimized and the energy used to keep the tanks at the desired temperature will be reduced. During the site visit, surface temperatures were measured and recorded for each of the uninsulated tanks. These values and the other significant variables affecting energy and cost savings are given in the table below.
**** INSERT TABLE FROM SPREADSHEET HERE ****
Anticipated Savings
The annual energy savings, ES, and energy cost savings, ECS, which could be realized by insulating tanks, can be estimated as follows:
where
HLb =heat loss rate from bare tank, Btu/h/ft2
HLi =heat loss rate from insulated tank, Btu/h/ft2
SF =uninsulated surface area, ft2
UH =annual time tanks are heated, h/yr
EFF =efficiency of the heat supply, from assessment measurements
C =conversion constant, 1,000,000 Btu/MMBtu
The 3E Plus Insulation Thickness Computer Program developed by the North American Insulation Manufacturers Association is used to determine the heat loss from bare and insulated pipes and flat surfaces. The 3E Plus program determines the heat loss from the tank surface using surface temperature, orientation, insulation properties and other data related to the heating system. A summary of the heating system and technical data used to determine the heat loss is presented in the table below.
Summary of Heating System and Technical Data
TECHNICAL DATAEmittance of Outer Jacketing / 0.10
Wind Speed / 8 mph
Emittance of Existing Surface / 0.80
As an example, ___ ft2 of the ____ tanks were measured to be ___ F during the site visit. The tanks are assumed to be heated for ____ h/yr. From the 3E Plus program, the heat loss from the bare surface of the tanks at ___F is ___ Btu/h/ft2. With 1" of fiberglass insulation, the estimated heat loss is ___ Btu/h/ft2. Thus,
The energy and cost savings for each of the tanks are listed in the table below.
**** INSERT TABLE FROM SPREADSHEET HERE ****
Summary of Energy and Cost Savings
As the table shows, the total energy savings are ____ ccf/yr and the corresponding cost savings are $____/yr.
Implementation Cost
The installed insulation costs[1] and simple payback periods are given in the table below.
**** INSERT TABLE FROM SPREADSHEET HERE ****
Insulation Costs
From the table, the total installation cost is $___. The cost savings of $____ will pay for the implementation cost in about __ months.
Equations for Heat Loss From Flat Surfaces
The 3E Plus Insulation Thickness Computer Program developed by the North American Insulation Manufacturers Association is used to determine the heat loss from the bare and insulated surfaces. For flat surfaces, the heat loss per unit area is given by the following equation:
where
Q/A =heat loss per unit surface area, Btu/h ft2
tP =process temperature, F
tS =outer surface temperature of insulation, F
R =thermal resistance of insulation subsurface (conduction only), F ft2 h/Btu
xi =thickness of the insulation layer, in
ki =thermal conductivity of the insulation layer, Btu in/h ft2F
Rs =thermal resistance of insulation surface, F ft2 h/Btu
ho =surface heat loss coefficient, Btu/F ft2 h
The conductivity is evaluated at the mean temperature of each layer. The surface resistance on the process temperature side is assumed to be sufficiently small in comparison to the other resistances so that the surface resistance can be neglected.
The surface resistance, Rs, represents the combined effects of radiation and convection heat loss. To calculate Rs, a variation of the Heilman equations is used. The equation is presented in ASTM C680. The combined convection and radiation heat loss from the tank surface is calculated as follows:
where
tA =ambient temperature, F
T =temperature in degrees Rankine, R = F + 460
ε =outer surface emittance, no units
σ =Stefan-Boltzman constant, 1.714 x 10-9 Btu/ft2 h R4
C =constant depending on surface orientation:
C = 0.7383 for vertical surface
C = 0.9480 for top of a hot tank or bottom of a cold tank
C = 0.4713 for bottom of a hot tank or top of a cold tank
V =wind velocity, mph
Since the surface resistance depends on surface temperature in a nonlinear manner, some type of iteration process is necessary. A back substitution method is used until the routine converges. Within the iteration loop, the thermal conductivity of the insulation material is continuously updated.
For the bare surface case, the surface temperature is set equal to the process temperature and it is not necessary to iterate for a surface energy balance.
The thermal conductivity k is calculated by integrating the thermal conductivity equation from the hot face temperature to the surface temperature. This requires an initial guess of the surface temperature. Each time the program iterates, the surface temperature is recalculated and a new thermal conductivity is calculated.
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[1]From R. W. Means Company, Inc., CostWorks 2000 Software, R. W. Means Company, Inc.: Kingston, Mass.