The Basics of Heat Calculations

The Basics of Heat Calculations

When selecting a process heater for industrial/manufacturing applications, one of the most common mistakes is making a selection based on its temperature rating rather than its power rating. Temperature can be important but is generally a dependant variable. In this article we will discuss why the power rating is such an important parameter and some of the fundamentals needed to calculate your process requirements.

When we "heat" something, we are really adding energy to a system in order to make the atoms move faster. Energy is expressed in the SI unit joules (other common units include BTU, calories, and kilowatt hours). Power is simply the amount of energy per unit time, or joules per second, and its SI unit is the watt. Therefore, the watt rating of a heater tells us how many joules of energy we can put into a system per second.

There are a myriad of factors that will impact the heat requirements of any process. Below are some of the design criteria you should consider. In any given application some of these criteria will be relevant and some will not (or will have such a small affect as to be negligible).

Calculating heat required to raise the temperature a given amount:
In order to raise any material from one temperature to another, energy must be added the system.

The calculation for this takes the form (in SI units):

Specific Heat of the Material	x	Mass of Material	x	Change in Temperature
kJ/(kg°C)	x	kg	x	°C

This formula provides the energy requirement in kilojoules for the temperature change. To calculate the power requirement in kilowatts, divide by the desired cycle time in seconds. The faster an object is heated, the more watts required. One way to cut down on the power requirements of a system is to ease the time constraints in the process.

Remember that you must take all materials into consideration. Include fixtures, trays, containers, etc. that may be used to hold the material of interest. They will also be brought to temperature during your process and therefore require added energy.

Calculating heat required to change from one state to another:

If your process requires the material to go from a solid to a liquid or a liquid to a gas, the energy calculation takes the form (in SI units):

Latent Heat of Phase Change (evaporation or melting)	x	Mass of Material
kJ/kg	x	kg

This formula provides the energy required in kilojoules for the phase change.

To calculate the power requirement in kilowatts, divide by time in seconds.

Calculating heat required to compensate for losses:
It is very difficult to build a heat system where no energy is lost to the surrounding environment. The amount lost depends on several factors including: thermal conductivity of the wall and insulation materials of the system, temperature differential between inside and outside of the system, and the thickness and surface area of the walls.

Thermal conductivity is the quantity of heat transmitted through a unit thickness in a direction normal to a surface of unit area, due to a unit temperature gradient under steady state conditions or W/m/°C.

The heat transfer rate is calculated using the following formula:

Thermal Conductivity	x	Surface Area	x	Temperature Difference/ Wall Thickness	=	Heat Transfer Rate
W/m/°C	x	m²	x	(T₁-T₂) °C/m	=	W

This is a simplified equation that assumes the system is at steady state and has flat—not curved–walls. Thermal loss calculations can become very complex and in some cases require computer simulation to get an accurate result, but a simplified equation such as the above can provide a good estimate.

Once you’ve calculated the above factors for your process requirements you have an idea of the size of heater you will require. If you find that the number of watts required is large, you should look at the factors in the calculations to see if there is an opportunity to mitigate. For example, if losses to the environment are causing a large increase in the requirements, look for ways to minimizes losses through insulation, process redesign, etc.

This article is meant to give a high level explanation of how to choose a heater based on its power output.

Sizing your Heater and Blower:

The Relationship between Air Flow and Temperature

Hot air systems, at their most basic, combine two components—a stream of air supplied by a blower/compressor and heat generated from a heating element—to produce hot air. Sometimes these two components are supplied by a single tool, other times they are supplied by two separate tools. The goal is to increase the temperature of the stream of air and to use this air for a task. Understanding how air flow and temperature relate to each other is helpful when choosing air and heat sources for your system to ensure your system will be able to do the work required.

Raising the temperature of air requires energy; the amount of energy required depends on the volume of air and the magnitude of the temperature increase. Air heaters are rated by power in watts or kilowatts which specify the energy that the element is capable of applying per unit of time. See our article on The Basics of Heat Calculations for further explanation.

There is an inverse relationship between air flow and temperature. For example, if the air flow over the element increases then one of two things happens: to maintain a constant temperature the element must increase its power output or the temperature of the output air will be lowered.

Why is this important?

This becomes important when choosing a heater and blower combination. Air heaters have a temperature-air flow curve associated with them as shown below:

Each heater has a curve, with power kept constant. This curve shows the maximum temperature the heater can attain at a given air flow. When designing a process heat system, this curve tells us the air flow range at which the heater can maintain a target temperature. It also indicates the minimum air flow required to prevent damage to the heater (i.e. the airflow at which the maximum rated temperature for the heater is reached). This chart allows us to size an appropriate blower for a heater in order to operate at our required temperature or to size a heater (in kW) for a required air flow.

Temperature versus Heat – What’s the Difference and Why is it Important?

By far the most common mistake we see is confusion between the concepts of temperature and heat. The terms are often used in conversation as if they are the same thing. This can have big implications when choosing a heater or designing a hot air system.

Heat and temperature are related. When heat is added to a material, the temperature of the material increases. The amount the temperature increases depends on the material and the amount of heat energy applied.

Let’s define the two concepts in greater depth:

Heat is form of energy. It can be transferred from one body to another. It can be created by, or transformed into, other forms of energy. As heat is a form of energy it can be used to accomplish useful work and it is measured in units of energy (SI units: Joules).

Temperature is the measurement of hot or cold. It is a measurable quality of a substance. Temperature cannot be “added” to a material and cannot do work. When an object’s temperature decreases, the object has lost heat energy to its surroundings; when its temperature increases, it has absorbed heat energy from its surroundings.

Why does it matter?

The distinction between temperature and heat is important to understand when selecting an air heater or designing a system.

Let’s look at an example:

An application requires a block of metal to be heated to 200°C in a given amount of time.

A common misperception is that any heater that gets to over 200°C will work for this application. Since all of the Leister heaters can reach 650°C any one of them should be able to do the job. This is the trap of designing by temperature rather than by heat.

The important parameter is actually the heat energy that is required to raise the temperature of the metal to 200°C.

This article provides an introduction to the basics of heat calculations.

Once the amount of heat energy (in joules) required to raise the temperature of the material is known, this information can be combined with the time limit (in seconds) to find the required power (in joules per second better known as watts) for the application. Once the number of watts is known, an appropriate air heater can be chosen based on its power rating. Remember that no process is 100% efficient, so some amount of expected loss should be included in your calculation.

Steps to Designing a Hot Air System – Part Four of Four

On the surface, designing an effective hot air system can seem like a simple exercise. However there is an underlying complexity which, when ignored, can result in wasted time and money. Designing a system without being aware of these complexities can often lead the incorrect conclusion regarding which tools should be used for an application.

Regularly, customers go to the STANMECH website and pick out a tool without going through the full design process. The best case scenario is that the customer lucks into selecting the correct tool. The worst case scenario is that the customer designs, builds, and commissions a complete processing line, spending a great deal of money and time to end up in a situation where it does not work. At this point they find themselves in a corner must spend more money and time to correct the problem.

The purpose of this series of articles is to keep the reader from going down the wrong path which, in most cases, can be avoided by thinking more carefully about the hot air system they are designing. Before continuing, be sure to read the first article which covers Defining the Problem, second which covers Gathering Process Information, and third which covers System Design before continuing.

In this article we will look at the next step – Selecting Process Equipment. Only after the first three steps are completed should we begin selecting equipment.

Steps to Designing a Hot Air System – Part Three of Four

The purpose of this series of articles is to keep the reader from going down the wrong path which, in most cases, can be avoided by thinking more carefully about the hot air system they are designing. Be sure to read the first and second articles which cover Defining the Problem and Gathering Information before continuing.

In this article we will look at the next step – System Design through Calculations, Experiments and Simulation. The results of this critical step determines the specifications of the equipment that should be installed for the application.

Steps to Designing a Hot Air System – Part Two of Four

Regularly, customers go to the STANMECH website and pick out a tool without going through the full design process. The best case scenario is that the customer lucks into selecting the correct tool. The worst case scenario is that the customer designs, builds, and commissions a complete processing line, spending a great deal of money and time to end up in a situation where it does not work. At this point they find themselves in a corner and must spend more money and time to correct the problem.

The purpose of this series of articles is to keep the reader from going down the wrong path which, in most cases, can be avoided by thinking more carefully about the hot air system they are designing. Be sure to read the first article which covers Defining the Problem before continuing.

In this article we will look at the next step – Gathering Process Information. This critical step builds on the problem definition by quantifying the important characteristics and constraints of the application.

Steps to Designing a Hot Air System – Part One of Four

On the surface, designing an effective hot air system can seem like a simple exercise. However there is an underlying complexity which, when ignored, can result in wasted time and money. At STANMECH we have been approached by customers who believe they know exactly what they need.

Customers go to the website and pick out a tool without going through the full design process. The best case scenario is that the customer wastes a small amount of money and time selecting and buying the wrong equipment. The worst case scenario is that the customer designs, builds, commissions a complete processing line, spends a great deal of money and time and end up in a situation where it doesn’t work, they’re in a corner, and much more money and time must be spent to correct the problem.

The purpose of this series of articles is to keep the reader from going down the wrong path when in most cases it can be avoided by more thinking more carefully about the hot air system they are designing.

In this article we are discussing the first step in designing a hot air system (or any system) – carefully defining the problem. It is tempting to skip right to a solution or jump into analysis without defining the problem you are trying to solve. Properly defining your problem will help to get to the appropriate solution and helps you work with third parties, such as equipment suppliers, more effectively.

Sizing your Heater and Blower:

The Relationship between Air Flow and Temperature

Raising the temperature of air requires energy; the amount of energy required depends on the volume of air and the magnitude of the temperature increase. Air heaters are rated by power in watts or kilowatts which specify the energy that the element is capable of applying per unit of time. See our article on The Basics of Heat Calculations for further explanation.

There is an inverse relationship between air flow and temperature. For example, if the air flow over the element increases then one of two things happens: to maintain a constant temperature the element must increase its power output or the temperature of the output air will be lowered.

Temperature versus Heat –

What’s the Difference and Why is it Important?

Let’s define the two concepts in greater depth:

Heat is form of energy. It can be transferred from one body to another. It can be created by, or transformed into, other forms of energy. As heat is a form of energy it can be used to accomplish useful work and it is measured in units of energy (SI units: Joules).

Temperature is the measurement of hot or cold. It is a measurable quality of a substance. Temperature cannot be “added” to a material and cannot do work. When an object’s temperature decreases, the object has lost heat energy to its surroundings; when its temperature increases, it has absorbed heat energy from its surroundings.

Rightsize your Heater: Don’t just buy the Biggest or Cheapest

Or – Why heaters come in different sizes

Every application requiring hot air has its own design requirements. Ideally, these requirements will guide the designer to the right heater for the job. Unfortunately, many times design decisions are made based on price or with a “bigger is better” mentality.

Looking at Leister’s line of air heaters you may notice that while the heaters range in power from 550W to 16kW they all have a maximum operating temperature of 650°C. We’ve written previously about the difference between temperature and heat and why the power rating of the heater is the critical specification when sizing a heater.

All-in-one Tools are not One-size-fits-all!

Or – Why a HOTWIND might not be the tool for you

Here at STANMECH, one of our most common customer requests is for the HOTWIND hot air blower. The HOTWIND is a well-designed combination heater/blower and it works extremely well in the right application. The attraction is obvious: it's everything you need in a compact package, it is capable of reaching the target temperature you require, and it looks more affordable because it’s only one unit rather than two.

However, the HOTWIND is often chosen for the wrong reasons. This type of tool incorporates a specific blower and a specific heater; unless the application lends itself to that exact blower and that exact heater size the tool is simply wrong for the application. This is true of all combination hot air blowers not just the HOTWIND.

Application Note:

Using a Leister LHS Air Heater with Compressed Air

When is it Appropriate to Use a Compressed Air Supply?

STANMECH typically recommends that Leister’s LHS air heaters be supplied with air by a blower. This is because a blower-based process heat system, when correctly sized and installed, will be far more energy efficient than an equivalent compressed air system. The associated cost-savings increase with the power rating of the heater—larger heaters require larger volumetric air flows resulting in more savings.

However, when broken down to its basics, LHS air heaters simply require a consistent supply of air flow regardless of its source. As long as the system is capable of providing an uninterrupted supply of air that meets the minimum flow requirements for the heater, the LHS air heaters can be operated safely using compressed air.

5 Biggest Mistakes Made When Designing a Hot Air System

1. Mixing up heat and temperature

The most commonly made mistake when designing a hot air system is the confusion of heat and temperature. While the terms are often used interchangeably, by definition they are very different. Heat relates to the amount of energy that is required in order to raise the temperature of a material or change its state (i.e. water changing from a liquid to a gas). Temperature is usually a specific design criteria or limitation, such as the melting point of a material or a temperature above which the material will degrade. When designing a process heat system both are important, however temperature is a design variable and heat is a calculated output.

2. Making equipment choices without sufficient investigation

Choosing the right equipment for a hot air system is a complex task that requires a thorough application analysis. Simplistic choices are often made without enough examination of the key design elements. For example, a system may be designed considering only the required target temperature while ignoring the amount of heat or type of air supply required. This results in the wrong equipment being specified and a system that does not work. Read our article about using hot air to create an oven as an example of the complexity of designing a hot air system.

Using Hot Air to Create an Oven

One of the most common application enquiries we get at STANMECH Technologies is about using air heaters to create an oven. Creating an oven is a logical use of air heaters but there are a few areas that require technical knowledge to ensure your oven achieves the desired end result. Below, we cover the 5 areas that require consideration when using hot air to create an oven.

1. The Air Volume Required

· In order to efficiently transfer heat to the objects in the oven, it needs to be continually flushed with hot air. At STANMECH, we use a rule of thumb of 10 flushes per minute but it could be less depending on the details of the design. Calculating the volume of 10 flushes per minute gives a conservative number for the air volume required by the blower, helps ensures good heat transfer, and limits problems like temperature gradients and heat loss to the environment.

· With air continually flushing the cavity venting becomes important. Intuitively people want to seal a system to eliminate air escaping to the environment but in this case you must ensure that the air has somewhere to go. The location of the venting is important and should be placed to encourage even distribution of the hot air in the chamber. We recommend partnering with a company that has experience creating hot air systems in order to ensure an even distribution of air within your oven.

· The volume of hot air required becomes a simple calculation:

Volume of the oven

Desired flushes per minute

Required air volume/minute

2. The Energy Required

· There are two major elements to the energy requirements of an oven:
1. Requirements to heat the flowing air to the desired temperature
2. Requirements to heat items in the oven to the required temperature

· There is a difference between heat energy and temperature. The temperature is a design criteria whereas the heat energy (usually stated in kilowatts) is the input. For more information on this topic see our article on The Basics of Heat Calculations.

· For details on how to calculate energy requirements see our page on Heat Requirement Calculations.

3. The Size and Shape of the Oven

· Ovens come in all shapes and sizes and this can impact the type and number of heaters required. Most of the complexity comes when ovens get large.

· The heater and blower choice becomes more important in larger ovens. Generally we recommend centrifugal blowers and open element heaters in order to generate the volume of hot air required. See our article on blowers and our whitepaper on heaters for more information.

· Temperature gradients and hotspots can be problematic and may have to be solved by using multiple heaters.

4. Temperature Control

· Each application has different requirements for how precisely the temperature needs to be controlled. The precision required changes the control method used. Tight acceptable temperature ranges will likely necessitate a closed loop control system. See our article Temperature Control of Air Heaters for more information.

· Precise control requires careful consideration of thermocouple placement. This is especially true in large ovens where several control zones may be required. For more information on thermocouple placement see our article Thermocouples in Hot Air Systems.

5. Air flow distribution

· Flowing air will want to move in a straight line unless forced to do otherwise. In order to achieve complete or even heating, there are times when mechanical solutions such as baffles or ducting must be used to redirect the hot air. We recommend seeking expert help with the design process to ensure your oven heats evenly.

Air heaters are an excellent candidate for building an oven but the design needs to be carefully considered. STANMECH Technologies has the expertise to make sure you get it right. Give us a call to discuss your application and ideas, we’ll help you figure out the next step in the design process and make sure you get equipment that will work for you.

Heat Requirement Calculations

When sizing the heaters for a particular application there are two energy requirements; the start-up heat and the operating heat.

Start-Up Heat is the heat energy required to bring a process up to operating temperature. Start-up heat requirement calculations, including material change of state, should be done in three parts.

1. Heat requirement from ambient temperature to change of state temperature.

2. Heat requirement during change of state.

3. Heat requirement from change of state temperature to operating temperature.

4. Operating Heat is the heat energy required to maintain the desired operating temperature through normal work cycles. A safety factor is usually added to allow for unknown or unexpected operating conditions. 10% is adequate for small systems with relatively few unknowns, while 20% additional wattage is more common, and figures of 25% to 35% may be considered for larger systems with many unknowns.

Start-up Heat Requirements

Heat requirements to heat up system

1. Wattage required to heat material:

	Weight of Material (lbs) X Specific Heat (Btu/lb °F) X Temperature Rise (°F)
Watts =
	3.412 Btu/watt hr. X Heat - up time (hr.)

2. Wattage required to heat container or tank:

	Weight of Container (lbs) X Specific Heat (Btu/lb °F) X Temperature Rise (°F)
Watts =
	3.412 Btu/watt hr. X Heat - up time (hr.)

3. Wattage required to heat hardware in container:

	Weight of Hardware(lbs) X Specific Heat (Btu/lb °F) X Temperature Rise (°F)
Watts =
	3.412 Btu/watt hr. X Heat - up time (hr.)

Specific Heat is the amount of heat required to change a unit mass of a substance by one degree in temperature. Specific Heat of Metals, Non-metals and Liquids and Gases can be found here.

Heat of Fusion – the amount of heat required to change a unit mass of a substance from solid to liquid without temperature change. Heat of fusion figures can be found here.

Heat of Vaporization - the amount of heat required to change a unit mass of a substance from liquid to vapor without temperature change. Heat of vaporization figures can be found here.

Heat Requirements for phase changes

4. Wattage required to melt a solid to a liquid at constant temperature

	Weight of Material to be Melted (lbs/hr) X Heat of Fusion (Btu/lb)
Watts =
	3.412 Btu/watt hr.

5. Wattage required to change a liquid to a vapor at constant temperature

	Weight of Material to be Vaporized (lbs/hr) X Heat of Vaporization (Btu/lb)
Watts =
	3.412 Btu/watt hr.

Heat Requirements to counteract surface losses.

Use Loss Rate graphs in the equations below.

6. Wattage to counter act liquid surface losses

	Total Liquid Surface Area (sq. ft.) X Loss Rate at Final Temperature (Watts/sq. ft.)
Watts =
	2

7. Wattage to counteract surface losses from container walls, platen surfaces, etc.

	Total Surface Area (sq. ft.) X Loss Rate at Final Temperature (Watts/sq. ft.)
Watts =
	2

Operating Heat

Heat Requirements to counteract losses

1. Wattage to counteract losses from open liquid surfaces:

Watts =

Total Liquid Surface Area (sq. ft.) X Loss Rate at Operating Temperature (Watts/sq. ft.)

2. Wattage to counteract container or platen surface losses, either insulated or uninsulated:

Watts =

Total Surface Area (sq. ft.) X Loss Rate at Operating Temperature (Watts/sq. ft.)

Heat Requirements to heat items transferred in and out of system

3. Wattage required to heat material transferred in and out of the system:

	Weight of Material to be Heated (lbs) X Specific Heat (Btu/lb °F) X Temperature Rise (°F)
Watts =
	3.412 Btu/watt hr. X Heat - up time (hr.)

4. Heat-up of racks of containers, etc. transferred in and out of the system:

	Weight of Items to be Heated (lbs) X Specific Heat (Btu/lb °F) X Temperature Rise (°F)
Watts =
	3.412 Btu/watt hr. X Heat - up time (hr.)

Regenerative versus Centrifugal Blowers:

What are they and when do I use what?

Both regenerative and centrifugal blowers are widely used in industrial processes. Superficially the two types of blowers can seem similar and it can be difficult to find good information on the differences between the two types and blowers and why one would be selected over the other.

First, we’ll discuss what the two types of blowers have in common. Both types move air using an impeller on a rotating shaft. The air comes in the inlet and is focused while traveling with the impeller before exhausted as linear flow at the outlet. This is where the similarities end.

How can you tell the difference?

The difference in construction between a centrifugal and regenerative blower make it easy to tell the two apart. A centrifugal blower is configured so that the inlet and outlet are perpendicular, with the inlet feeding air into the centre of the impeller and the outlet tangential to the rotation of the impeller (picture). In a regenerative blower the inlet and outlet are parallel with both positioned perpendicular to the rotation of the impeller.

What is the real difference?

The impeller and impeller casing of the two types of blowers are designed for different end uses. In centrifugal blowers, air enters at the center of a rotating impeller on which there are a number of fixed vanes. Through centrifugal action, air is forced to the periphery of the impeller and housing where it is discharged as a steady stream through the outlet. The negative pressure created at the centre hub, in turn, sucks in more air. The vanes basically act as paddles to push volumes of air to the outlet. No significant pressure is built up in this process but centrifugal blowers can be used to move large volumes of air.

In regenerative blowers, as the impellor pushes the air around the ring, centrifugal forces cause the air trapped between the rotating impeller vanes to move towards the blower casing. The air flow is then forced to the base of a following impeller vane for recirculation in the same manner. This circular flow in combination with the revolution of the impeller causes air to follow a spiral path through a regenerative blower (see picture); the result is air that is under constant acceleration.

This "regeneration" of air with each revolution allow regenerative blowers to develop significant pressure.

As a general rule centrifugal blowers are considered low pressure, high flow blowers while regenerative blowers are high pressure, low flow blowers. These fundamental characteristics inform the types of applications to which they are suited.

Where to use what?

Centrifugal blowers work for applications such as heating a furnace with hot air where a large air volume is required to fill the space. Blow off applications generally are better suited to centrifugal blowers because large volumes of air are required to get optimum air velocity out of air knives.

Regenerative blowers can produce enough pressure to overcome air flow constraints in a process system; for example, if a nozzle is required that constricts the air flow from a large cross section to a smaller cross section. If the air is required to travel a sub optimal path with sharp bends or a torturous route the pressure of a regenerative blower will be useful.

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