Water heaters are used to transform thermal energy from a fuel source to a stream or reservoir of water. They can be described by their fuel source or may be one of several specific types of heaters. They can be configured to operate in-line, to heat re-circulated streams of water, for direct immersion in an open tank or vessel, or to operate as a booster heater to increase the fluid stream temperature at or near its point of use.

The selection of water heaters should be done on the basis of their type, application and heating capacity. Other important specifications to consider include the energy source and ratings.

Types of Water Heaters

Water heaters can be classified for industrial, commercial or residential use.

Industrial Water Heaters

Industrial water heaters are used in a variety of direct and indirect heating applications. Indirect heating is accomplished by using water as a thermal fluid. Convection allows the heat absorbed to be transferred to a closed system. Direct heating of water is used when hot water is a resource used in an industrial application. Common types of industrial water heaters are circulation heaters, immersion heaters and over-the-side heaters.

Immersion Heaters	Immersion heaters are used in applications that require immersing the heater in the substance to be heated.	Image Credit: Alstrom
Circulation Heaters	Circulation heaters are used primarily to heat moving, flowing, or circulating fluid streams. Fluid flows through the heater, which transfers heat to the stream.	Image Credit: Chromalox
Over-the-Side Heaters	Over-the-side heaters are immersion heaters that hang over the side of a tank of heated material.	Image Credit: Chromalox

Residential and Commercial Water Heaters

Commercial water heaters are gas, oil or electric-fired devices used to heat water for residential and commercial use. The most common types are storage water heaters and demand water heaters.

Storage water heaters consist of an insulated vessel or tank coupled with a thermostat and an immersion heating element or other heat source. They work by heating up water inside the insulated tank to a controlled temperature. The use of a heated reservoir makes larger quantities of hot water readily available.

• Advantages: fuel flexibility; high flow capacity
• Disadvantages: standby energy loss

On demand heaters provide hot water without a storage tank. An on demand water heater uses gas, electricity, or propane as the heat source to heat the water and reduces the energy consumption by eliminating the standby losses caused by unused hot water in a tank. An on demand water heater is also known as a tankless water heater. A tankless hot water heater constantly delivers hot water by activating its heating device with the flow of water.

• Advantages: energy-efficient; smaller footprint
• Disadvantages: delay time; flow rate limited by heater output

There are many other types of water heaters available. Examples include electric water heaters, solar water heaters, and gas water heaters.

Electric water heaters are electrically powered devices and may operate as an on demand heater or as a storage tank heater. Atypical electric storage tank heater has two elements, separated by a thermostat. One of the elements is placed on the top of a tank and the other at bottom. The top element heats water and when the water reaches the correct temperature, the top element shuts off and the lower element heats the remaining water.

Solar water heaters concentrate the sun's thermal energy to heat water for industrial, commercial, and residential use. There are two main types of products: active and passive. Active solar water heaters have circulating pumps and controls. Passive products do not. Both types of solar water heaters mount on roofs or walls and have storage tanks and solar collectors.

Gas water heaters have a combustion chamber and use either natural gas or liquid propane (LPN). Although they are less efficient than electric water heaters, they are widely used in commercial and residential applications due to the economics of the energy source and the heat transfer rate. Like electric water heaters, they can operate with or without a storage tank.

Applications

When selecting a water heater it is important to select a heater that is designed for its intended application. The heated medium, water, can be classified into more succinct categories such as potable or clean water, ultrapure water or process water.

Potable (Clean) Water

Clean water, or more specifically potable water, is water that meets or exceeds the guidelines set by public authority and is safe for human consumption. In the United States, the National Primary Drinking Water Regulations (NPDWRs) has set standards that describe the maximum concentration of known contaminants that can be present without there being an expected health risk. Clean water may contain minerals, acceptable background concentration of contaminates or, when dealing with public water supplies, fluoride which is added to prevent tooth decay. Clean water can be considered the least reactive and least corrosive type of water. Heaters used for clean water may use copper heating elements and steel walled vessels.

Ultrapure Water

Ultrapure water is deionized water from which background levels of contaminants have been removed. Water is the ideal solvent and when impurities are removed, reactivity increases. Heaters designed for ultrapure water applications include stainless steel tanks and nickel alloy sheathed heating elements in order to prevent corrosion and contamination of the water supply. Industries that use ultra pure water include semiconductor manufacturing and pharmaceuticals.

Process Water

Process water is a working fluid used in manufacturing processes. It may be used as a cleaning medium, as a coolant, or even as a transport mechanism. In each case there is a possibility for additional impurities such as contaminants, soaps or detergents to become dissolved in the fluid. These impurities may increase the corrosive nature of water, creating a need for improved vessels and heating elements materials that are resistant to oxidation and corrosion. 

Heating Capacity

Heating capacity is a measurement of the amount of thermal energy that a water heater can deliver. In order to optimize efficiency, water heaters are carefully sized for their intended application. The two most important factors needed to size the capacity of a water heater are the desired flow rate and temperature rise.

hout = q ρ dt cp

Where:

hout = heating capacity, output (Btu/h)

q = flow rate (gph)

ρ = 8.34 - density of water (lbs/gal)

dt = temperature rise (^oF)

cp = 1.0 - specific heat capacity of water (Btu/lb^oF) 

Formula Credit: The Engineering ToolBox

Energy Sources

Water heaters are primarily powered by electricity, combustible fluids, thermal fluids, solar energy or combustible solids (e.g. wood, wood pellets).

Electrically Powered

Electrically powered industrial heaters may use either single-phase or three-phase power. Typically, these devices use alternating current (AC) instead of direct current (DC) and differ in terms of watt density. When listed as a range or maximum amount, watt density provides a good measure of how quickly the industrial heater can transfer heat.

Image Credit: http://www.htwtr.com

Combustible Fluids

Combustible fluids used to power industrial heaters are composed of simple or complex hydrocarbons. They can be further classified into two subgroups related to the stable physical state of the substance at standard temperature and pressure. The two physical states of matter observed are either a liquid or gaseous state. Combustible fluids that exist in a liquid state at standard temperature and pressure include diesel, fuel oil, gasoline and kerosene. Combustible fluids that exist in a gaseous state at standard temperature and pressure include natural gas and propane.

Image Credit: Eberspächer UK Ltd

Thermal Fluids

Thermal fluids are used to carry thermal energy in process heating applications. In these applications the thermal fluid is used to convect thermal energy from a heat source to a process, system or closed environment. Steam and hot water are the two most common types of thermal fluid used in industrial heating applications.

Image Credit: Alfa Laval

Solar Energy

Solar energy is a renewable energy source that can be used to operate or supply thermal energy to a system by the use of a solar collector and a thermal mass transfer system.

Image Credit: DOYOURBIT.net

Combustible Solids

The primary power source burns wood, wood pellets, or other combustible solid products.

Image Credit: Science Learning Hub

Energy Source Ratings

Federal regulations require that water heaters adhere to Energy Guide Labels which include Energy Factor (EF) and First Hour Rating (FHR).

The Energy Factor (EF) is a number from 0 to 1 with two significant figures used to describe the efficiency of the heater. The EF compares power consumption and heating capacity.

The First Hour Rating (FHR) indicates the amount of water the heater will produce during peak hour usage.

Resources

Popular Mechanics - How It Works: Water Heater

EPA - Drinking Water Contaminants

Energy.gov - Water Heating

Chilipepper Sales - Gas Water Heaters

Image credit:

Alfa Laval Inc.