How to Size a PTFE Heater for a Tank with a Floating Plastic Cover for Fume Suppression?

They are wonderful ways to suppress fumes and save energy. Adding a floating cover to an open tank is a terrific method to reduce fume and save energy. However, if you're calculating wattage for a new heater for a covered tank, the normal rule of thumb of "watts-per-gallon" becomes too conservative, leaving you with a large and expensive heater. The addition of a floating plastic cover comprised of hollow plastic balls, a solid foam sheet or floating tiles dramatically changes the thermal behaviour of an open liquid bath. The lid inhibits evaporation, the principal mode of heat loss for the majority of open industrial tanks. Still the heater must be sized for two different operating circumstances initial heat up and steady state maintenance. In order to properly size PTFE heaters for tank floating cover applications it is necessary to understand how the cover impacts each condition.

Why Open Tanks Lose Heat Faster Than Covered Tanks - The Physics
Exposed tanks lose heat through three major mechanisms:

Evaporation: Molecules go away from the surface of the liquid, taking away the latent heat of vaporisation. This is generally the biggest loss, especially at elevated temperatures (e.g. 60–90 °C).

Convection : The hot air over the liquid goes up , carrying heat away from the surface .

Radiation and conduction via tank walls: Heat radiates from the surface of the liquid and is conducted through the tank walls and bottom.

A floating cover provides a barrier between the liquid surface and the atmosphere and greatly lowers evaporative loss. Most floating covers also have some insulating value so minimising convective and radiative losses from the surface. Depending on cover design and fit, overall heat loss from a covered tank can in fact be reduced by 50-70% compared to an open tank. However the cover does not modify the heat required to raise the temperature of the bulk liquid from cold start to operational temperature.

Cover does not affect Heat-Up Power
The initial heating power depends only on:

Mass of liquid (in kg or lb)

The specific heat of the liquid (for water based solutions, somewhere about 4.18 kJ/kg °C or 1 BTU/lb °F)

The required temperature rise (ambient to setpoint, ΔT)

The ramp time allowed (hours or minutes)

Or, in practical numbers, for water about 2.5 watt-hours per gallon per °F (0.66 Wh/L·°C), neglecting heat losses in the heating. Add in losses (as they always should be) and you are looking at about 3-4 W/gal-°F for an uninsulated open tank. A floating cover makes no difference to this computation. This is because at the beginning of the heat-up the surface of the liquid is cold and evaporative losses are small until the temperature near the setpoint. More importantly, the lid does not diminish the amount of energy that has to be put into the liquid mass. So the electricity consumption to heat up is precisely the same as for an uncovered tank.

Maintenance Power: Drastically Decreased
When the tank is at operational temperature, the heater has to do simply the work of replacing heat lost to the surroundings. In an open tank the evaporative loss prevails. A floating cover virtually eliminates evaporative loss (if it is fully sealed), and decreases convective and radiative surface losses. The rest of the losses are borne by:

Walls and bottom of tank (conduction)

The floating cover itself (conduction through plastic)

Any space between the cover and tank walls

In practice, the maintenance power for a covered tank can be estimated as 30-50% of the open tank maintenance value. For example, a common water based pickling or cleaning bath at 60-70°C would have open tank maintenance of perhaps 20-30 W/ft^2 of liquid surface, plus wall losses. With a floating plastic ball cover, the surface losses are reduced to 5-10 W/ft2. A solid floating foam sheet can be lower still.

How to Size a PTFE Heater for a Covered Tank
The right sizing methodology is based on 2 computations and the greater of the 2 is used to calculate the installed watts.

Step 1: Calculate Heat-Up Power (Must be done, cover or no cover)
Calculate the liquid volume in gallons or litres

Multiply by density to get mass.

Select a ramp time that is realistic (usually 2–4 hours for the majority of industrial tanks).

Determine the power needed to heat the liquid from ambient (e.g. 20°C) to setpoint (e.g. 70°C) in that time, using the specific heat of the liquid.

Add a percentage for losses during heat-up (about 10-20% for a covered tank during heat-up, as the surface is still losing some heat as the temperature rises).

A 500 gallon water tank (≈ 4000 lb) has to be heated from 20°C to 70°C (ΔT = 50°C = 90°F) in 3 hours. In theory, the energy is 4000 lb × 90°F × 1 BTU/lb·°F = 360,000 BTU. 360,000 BTU ÷ 3412 BTU/kWh = 105.5 kWh, or convert to kW. Average Power = 35.2 kW over 3+ hours. Add 15% for losses and you get about 40 kW heating power.

Step 2: Calculate Maintenance Power (Less cover)
Estimate the total heat loss from the covered tank at the setpoint. This comprises:

Floating cover surface loss (use manufacturer data or engineering estimates: ~5–10 W/ft^2 for plastic balls, ~3–6 W/ft^2 for solid foam sheet at 20°C air-liq. temperature difference)

Wall and bottom losses (not dependent on cover, usually 5 to 15 W/ft² of exterior tank surface area)

Add these losses together to get the steady state maintenance watts.

For example, a 500-gallon tank has a liquid surface area of 20 ft². Floating plastic balls, surface loss = 8 W/ft 2 = 160 W. Wall area (not including top) = 80 ft 2 Wall loss = 10 W/ft 2 = 800 W Maintenance Total = 960 W (≈1 kW). Even with a conservative safety factor of 2, maintenance is still just 2kW.

Step 3: Comparison and Selection of Installed Wattage
The heat-up power (40 kW) is significantly larger than the maintenance power (1-2 kW). The installed wattage of the PTFE heater must be at least 40 kW for the required heat-up time. The cover does not lessen this requirement. Once the tank is at temperature, however, the heater will turn off or run at a very low duty cycle, drawing only the 1-2 kW needed to make up the losses. This is a considerable energy reduction compared to an open tank which may require 4-6 kW for maintenance.

Adding a cover makes the tank a better steady-state system, but it does not change the thermodynamics of heating it up from cold. Sizing based on maintenance power alone would provide a heater that would be able to hold temperature, but would take an unreasonably long time (several hours or days) to reach setpoint from a cold start.

Practical PTFE Heater Sizing Guidance
PTFE heater size tank floating cover application: The following actions are recommended:

Always calculate heat-up power first using the tank volume, required ramp time and liquid specific heat. This will almost always influence the total wattage installed.

Estimate operating costs and by calculating the maintenance power with the cover on, make sure that the lowest controllable output of the heater (if using a power controller) can be lowered down enough to prevent overshoot of temperature.

Do not reduce the heat wattage due to the cover. This means slow heat-up, late production start and unhappy customers.

Use several little heaters rather than one large heater . Better temperature uniformity and redundancy . This is especially true in tanks with covers, because the cover may hinder natural convection currents.

The PTFE heater should be below the floating cover, usually at the bottom of the tank for good circulation. Do not allow the cover to touch the heater.

Special Considerations : Cycling and Thermostat Selection
The installed heater capacity is 40 kW, and the maintenance load can be as low as 1–2 kW. Therefore, a normal on-off thermostat with a significant differential may generate too high temperature swings (overshoot and undershoot). A PID controller is recommended or a proportional power regulator (SCR). These devices ramp down power as the setpoint is reached, so the big 40 kW element does n't pour full heat into a tank that needs only 2 kW of replacement energy .

Summary
Use of floating plastic cover is an effective energy saving strategy for open tanks as it reduces evaporative heat loss and formation of fumes. Regarding heater sizing, the cover greatly reduces the maintenance power need, but it does not reduce the heat-up power requirements. The wattage of the installed PTFE heater still needs to be based on the energy required to heat the entire volume of liquid from ambient to operating temperature within an acceptable period. The major benefit is after heat-up, when the heater cycles down to a fraction of its installed capacity, substantially reducing operating expenses. Energy saving measures should be incorporated into the sizing process, not by reducing the installed heater size but by choosing the suitable control systems that take advantage of the lower steady-state demand. A properly sized tank will heat rapidly when called upon and operate efficiently beneath the cover, providing productivity and energy savings.