ENERGY LOST AND COST

Energy lost and cost



Energy lost and cost

Question

Analyze heat lost from a 6-inch OD steam line. The line carries high-pressure steam throughout the factory at 400 psi, 400 oF. Specify type and thickness for insulating the steam line. Compare the cost of energy lost with and without pipe insulation. You can assume steam is generated 8000 hours/year. Compare energy cost reductions based upon a gas fired boiler and a fuel oil fired boiler. Assume boiler energy input at 6.5(106) Btu/hr.

Solution

Calculation of Insulation Thickness

The most basic model for insulation on a pipe is shown below. r1 show the outside radius of the pipe r2 shows the radius of the Pipe+ insulation.

Figure 1

Insulated pipe section

Heat loss from a surface is expressed as

H = h x A x (Th-Ta) ……… (1)

Where,

h = Heat transfer coefficient, W/m2-K

H = Heat loss, Watts

Ta = Average ambient temperature, K

Ts = Desired/actual insulation surface temperature, ºC

Th = Hot surface temperature (for hot fluid piping), ºC & Cold surface temperature for cold fluids piping).

For horizontal pipes, heat transfer coefficient can be calculated by:

h = [A + 0.005 (Th - Ta)] x 10 W/m2-K

For vertical pipes,

h = (B + 0.009 (Th - Ta)) x 10 W/m2-K

Using the coefficients A, B given below:

Coefficients A, B for estimating 'h' (in W/m2-K)

Table 1

Tm =

k = Thermal conductivity of insulation at mean temperature of Tm, W/m-C

tk = Thickness of insulation, mm

r1 = Actual outer radius of pipe, mm

r2 = (r1 + tk)

Rs = Surface thermal resistance = ºC-m2/W

Ri = Thermal resistance of insulation = ºC-m2/W

The heat flow from the pipe surface and the ambient can be expressed as follows:

H = Heat flow, Watts

= = ……….. (2)

From the above equation, and for desired Ts, Rl can be calculated. From Rl and known value of thermal conductivity k, thickness of insulation can be calculated.

Equivalent thickness of insulation for pipe, Etk = (r1 + tk) x ln ()

Insulation material

Protection materials are arranged into natural and inorganic sorts. Natural isolations are dependent upon hydrocarbon polymers, which could be unfolded to get high void structures. Cases are thermocol (Expanded Polystyrene) and Poly Urethane Form (Puf). Inorganic cover is dependent upon Siliceous/aluminous/calcium materials in stringy, granular or powder structures. Samples are Mineral fleece, Calcium silicate and so forth. Lands of regular covering materials are as under:

Calcium Silicate: Used in modern process plant funneling where high administration temperature and compressive quality are required. Temperature goes ...