Making Decisions with Insulation

Many people overlook the importance of insulation in the chemical industry. Some estimates have predicted that insulation in U.S. industry alone saves approximately 200 million barrels of oil every year. While placing

insulation onto a pipe is fairly easy, resolving issues such as what type of insulation to use and how much is not so easy. Insulation is available in nearly any material imaginable. The most important characteristics of any insulation material include a low thermal conductivity, low tendency toward absorbing water, and of course the material should be inexpensive. In the chemical industry, the most common insulators are various types of calcium silicate or fiberglass. Calcium silicate is generally more appropriate for temperatures above 225⁰C (437 ⁰F), while fiberglass is generally used at temperatures below 225 ⁰C.

A Brief Look at Theory

The most basic model for insulation on a pipe is shown below. R1 and R2 show the inside and outside radius of the pipe respectively. R3 shows the radius of the insulation. Typically when dealing with insulations, engineers must be concerned with linear heat loss or heat loss per unit length.

Generally, the heat transfer coefficient of ambient air is 40 W/m² K. This coefficient can of course increase with wind velocity if the pipe is outside. A good estimate for an outdoor air coefficient in warm climates with wind speeds under 15 mph is around 50 W/m² K. The total heat loss per unit length can then be calculated by:

(2)

Since heat loss through insulation is a conductive heat transfer, there are instances when adding insulation actually increases heat loss. The thickness at which insulation begins to decrease heat loss is described as the critical thickness. Since the critical thickness is almost always a few millimeters, it is seldom (if ever) an issue for piping. Critical thickness is a concern however in insulating wires. Figure 3 shows the heat loss vs. insulation thickness for a typical insulation. It's easy to see why wire insulation is kept to a minimum as adding insulation would increase the heat transfer.

Thinking About Insulation from All Sides

Three major factors play an important role in determining insulation type and thickness. Here, we'll focus on resolving the thickness issue since many manufacturing facilities have a "standard" type of insulation that they use. The three key factors to examine are:

1. Economics

2. Safety

3. Process Conditions

Each situation must be studied to determine how to meet each one of these criteria. First, we'll examine each aspect individually, then we'll see how to consider all three for an example.

Economics

Economic thickness of insulation is a well docu

mented calculation procedure. The calculations typically take in the entire scope of the installation including plant depreciation to wind speed. Data ch

arts for calculating the economic thickness of insulation are widely available. Below are links to economic thickness tables that have been adapted from Perry's Chemical Engin

eers' Handbook:

Table 1: Economic Indoor Insulation Thickness (American Units)

Table 2: Economic Indoor Insulation Thickness (Metric Units)

Table 3: Economic Outdoor Insulation Thickness (American Units)

Table 4: Economic Outdoor Insulation Thickness (Metric Units)

A small, DOS based computer program is also available to help determine econ

omic thickness of insulation. this program here! (Also available in Software Corner)

Example of Economic Thickness Determination:

Using the tables above, assuming a 6.0 in pipe at 500 ⁰F in an in

door setting with an energy cost of $5.00/million Btu, what is the economic thickness?

Answer: Finding the corresponding block to 6.0 in pipe and $5.00/million Btu energy costs, we see temperatures of 250 ⁰F, 600 ⁰F, 650 ⁰F, and 850 ⁰F. Since our temperature does not meet 600 ⁰F, we use the thickness before it. In this case, 250 ⁰F or 1 1/2 inches

of insulation. At 600 ⁰F, we would increase to 2.0 inches of insulation.

Economic thickness charts from other sources will work in much the same way as this example.

Safety

Pipes that are readily accessible by workers are subject to safety constraints. The recommended safe "touch" temperature range is from 130 ⁰F to 150 ⁰F (54.4 ⁰C to 65.5 ⁰C). Insulation calculations should aim to keep the outside temperature of the insulation around 140 ⁰F (60 ⁰C). An additional tool employed to help meet this goal is aluminum covering wrapped around the outside of the insulation. Aluminum's thermal conductivity of 209 W/m K does not offer much resistance to heat transfer, but it does act as another resistance while also holding the insulation in place. Typical thickness of aluminum used for this purpose ranges from 0.2 mm to 0.4 mm. The addition of aluminum adds another resistance term to Equation 1 when calculating the total heat loss:

However, when considering safety, engineers need a quick way to calculate the surface temperature that will come into contact with the workers. This can be done with equations or the use of charts. We start by looking at another diagram:

At steady state, the heat transfer rate will be the same for each layer:

Rearranging Equation 4 by solving the three expressions for the temperature difference yields:

Since the heat loss is constant for each layer, use Equation 4 to calculate Q from the bare pipe, then solve Equation 6 for T4 (surface temperature). Use the economic thickness of your insulation as a basis for your calculation, after all, if the most affordable layer of insulation is safe, that's the one you'd want to use. If the economic thickness results in too high a surface temperature, repeat the calculation by increasing the insulation thickness by 1/2 inch each time until a safe touch temperature is reached.

As you can see, using heat balance equations is certainly a valid means o

f estimating surface temperatures, but it may not always be the fastest. Charts are available that utilize a characteristic called "equivalent thickness" to simplify the heat balance equations. This correlation also uses the surface resistance of the outer covering of the pipe. Figure 4 shows the equivalent thickness chart for calcium silicate insulation. T

able 5 shows surface resistances for three popular covering materials for

insulation: