Heat Trace Cable Selection Requires Attention to Details
- Calculate pipe or tank dimensions.
- Specify insulation type, thickness and K factor.
- Specify pertinent temperatures (ambient, startup, maintenance, etc.).
- Describe the operating/ambient environment.
- Calculate heat loss from the pipe or tank.
- Select the appropriate cable type and output rating.
- Specify the cable length needed.
- Specify/verify controller and electrical power distribution specifications.
- Complete the bill of materials with electrical connection and mechanical attachment hardware.
Pipe/Tank Dimensions. Pipe and tank dimensions are used both to calculate heat loss and to determine the length of cable needed. For pipes, you must purchase enough for the pipe run, plus additional length to go around inline components. For the surface area of a tank, use one of the following equations:
Cylindrical Tank Area
SAC = 2Lr2 + LDH
where r is the radius, D is the diameter and H is the tank height.
Rectangular Tank Area
SAR = 2[(W x L) + (L x H) + (H x W)]
where L is length, W is width and H is tank height.
Insulation. You also need to know pipe or tank insulation thickness and its K-factor to calculate heat loss. Table 1 provides K-factors at various temperatures for commonly used insulation materials.
Specify Pertinent Temperatures. As part of your heat trace system design, you must specify a few temperatures. These temperature values are used in heat loss calculations and in determining the type of cable to use. The pertinent values are:
- Maintenance temperature of the pipe or tank (Tm).
- Minimum startup temperature (Ts).
- Minimum ambient air temperature(Ta).
- Maximum exposure temperature for the cable with no power applied.
Operating/Ambient Environment. Besides the temperatures mentioned, the heat trace cable jacket material must be selected for environmental exposure factors. In order of increasing severity, these are moisture (high humidity or water in liquid form); aqueous solutions of inorganic compounds; and liquid organic chemicals, acids and bases. The specific chemicals and conditions must be known so that appropriate jacket material can be selected, according to manufacturer recommendations.
Calculate Heat Loss from the Pipe or Tank. The amount of heat loss from the pipe or tank is the most critical calculation - it determines the length and wattage output of the heat trace cable. The steps below will help you avoid mistakes and select only as much cable and output wattage as you need.
- 1. Calculate Temperature Difference
gT = Tm - Ta
2. Locate the Appropriate Heat Loss (Qp or Qt) for your insulated pipe or tank (table 2).
3. Calculate Unadjusted Heat Loss
a. Pipe: Q = Qp x gT watts per lineal foot of pipe
b. Tank: Q = Qt x gT watts per square foot of tank surface
a. Insulation K-Factor QK = Q x 2(KTm + KTa) where table 1 is used to find the K-factors at temperature values equal to or less than Tm and Ta. b. Wind Speed Adjust QK for higher wind speeds than the 20 mph assumed in table 2.
Qw = QK x 1.05 for 25-30 mph wind speed, or
Qw = QK x 1.10 for 30 mph and above wind speeds.
c. Inside Installation
QI = 0.9 Qw if installation is indoors d.Specification Safety Factors Multiply QI or Qw by additional safety factors required by specifications.
5. Calculate Overall Heat Loss
a. Pipe: As appropriate, QOP = QI or QOP = Qw watts per lineal foot
b. Tank: As appropriate, QOT = SA x QI or QOT = SA x Qw watts per lineal foot
Select Cable Type and Output Rating. The two varieties of heat trace cable most often used are the constant wattage type and self-regulating type. Typically, constant wattage cable is used for metal piping while self-regulating cable is used for both metal and plastic pipes. Different manufacturers may offer different temperature ratings, power outputs and jacket materials, but virtually all heat trace comes with tinned copper braid for grounding. Tables 3 and 4 list typical temperature specifications.
When using self-regulating cable, you must use the manufacturer's graphs or charts for selecting the appropriate output rating. The example in figure 1 illustrates output selection for plastic piping. (Follow the manufacturer's selection instructions for metal piping and tanks, or plastic tanks.)
In this example, assume that the self-regulating cable is installed on plastic piping and over-wrapped with aluminum tape. The pipe maintenance temperature is assumed to be 100°F (38°C) and the calculation of overall heat loss is 4 W/ft. The manufacturer's curves for this type of installation appear in figure 1. A vertical line is drawn upward from 100°F on the X-axis. A horizontal line is drawn from the heat loss value of 4 W/ft until it intersects the vertical line you drew. Select the cable with the wattage rating curve that lies above the intersection of the two lines - that is, the cable rated 8 W/ft.
Specify Cable Length. For pipes, you need a cable length equal to the pipe run plus additional length to allow for inline components. Table 5 lists typical length allowances for piping components.
For tanks, a length is selected that generates a power output equal to the wattage value found for the tank in Step 5 of the heat loss calculations. The length required is calculated as
L = QOT ÷ W/ft
where the last term in the equation is the cable output in watts per foot. This will be based on the cable type and rating selected for the tank in the section above.
Specify/Verify Controller and Power Distribution Specifications. This is beyond the scope of cable selection, but it is an important step in completing a heat trace system design. Depending on application, you may need on/off, PID or smart tuning as well as temperature alarm options such as process deviation, band, high/low and latching/non-latching (manual or automatic reset). You probably also will want an adjustable low current alarm, which warns you if current flow through the cable is reduced to an unacceptable value.
A key issue is whether to use an ambient air or a pipe wall temperature sensor. For pipe freeze protection, ambient air sensing and on/off control usually are adequate, but for process applications, this method may allow excessive temperature swings. Pipe wall sensing with loop control provides a narrower temperature band. A controller with a data communication interface such as RS422 or RS485 may be required to allow remote monitoring of the temperature and alarm conditions.
Because National Electric Code (NEC) requires a ground fault interrupt on heat trace cable installations, the controller should provide a GFI trip alarm option. This should be adjustable over a range that covers the trip value listed in the applicable NEC standard.
Power distribution to the heat trace cable circuit is best supplied through a thermal magnetic style circuit breaker with integral GFI. The GFI usually is set at 30 mA to avoid nuisance tripping at lower temperatures. Size the circuit breaker for 25% more current than you calculate is needed based on the total heat trace cable load. Be sure to follow the manufacturer's recommendations for maximum circuit length.
Bill of Materials. Ultimately, the bill of materials for a complete heat trace system will include the model numbers, description and quantity or length for the following items: heat trace cable; controller; power, splice/tee and end seal connections; and attachment devices such as aluminum or electrical tape, cable ties or pipe straps; and caution labels.
Although this whole process involves a lot of detail, carefully specifying a heat tracing system will provide you with a system that will operate effectively.