by: John Baudreau, Herb Elliott, Gary Dixon
Pages: S10-S12; October, 2001

 

1. Evaporator maintenance (removal of accumulated residues, cleaning of mist eliminators, heat exchangers, mid-process oil removal, and/or settled solids blowdown)

2. Evaporation process upsets (due to variations in wastewater oil loading, solids loading, or foam formation during evaporation).

3. Periodic evaporator operator training and preventive maintenance (on the evaporator itself).

4. Normal operating energy cost (based on fuel consumption required). No evaporator can perform at 100 percent efficiency.

Evaporator Maintenance

Evaluation of maintenance cost is best done by obtaining the evaporator manufacturer’s estimate of maintenance on their system when processing your waste. Obtain figures for length of each maintenance operation and its frequency. Together, the five maintenance operations (see #1 above) can be significant. But when the evaporator design properly anticipates the user’s wastewater characteristics, this total can be very low. Nonetheless, you should know it. The manufacturer’s assistance here is invaluable.

Process Upsets

Evaporation process upsets are harder to quantify; but, again, the evaporator manufacturer’s estimates are key to appraising this cost element based on your wastewater’s estimated range of oil and solids loading. On the other hand, only actual testing of your typical waste will be appropriate in assessing the impact of foam, a process upset that can severely limit your wastewater throughput.

This may require, (a) adjusting heat input to the system, (b) adjusting normal liquid operating level within the evaporator, and/or (c) adding chemical antifoam chemistry periodically. All of these obviously impact operating cost but potentially most significant is the requirement for antifoam chemistry. Be sure your evaporator supplier provides a foam assessment and recommends a specific chemical and dosage rate to permit your evaluation of this cost element, which very much depends on the evaporator design.

Training and PM

Wastewater evaporators generally take little time to start and operate. New operators generally become comfortable with very little training. However, preventive maintenance (PM) requires more effort.

Since the evaporator is frequently considered a "garbage can" for liquids it can be badly abused. Without sufficient control over what is placed in it, what conditions need monitoring, and—most of all — how often residues must be removed, the evaporator can become subject to extensive downtime for correction of easily avoidable conditions. Most common among user concerns are conditions caused by over-concentrating non-water constituents in the waste. If you were evaporating only water (instead of wastewater) there would be little concern. But then also there would likely be no need for an evaporator. It’s the presence of waste contaminants that create wastewater that is unacceptable to the sewer

As with items 1 and 2 above, these training and PM costs must be discussed with the evaporator supplier to assess likely hours that will be consumed in establishing good evaporator habits. Service agreements with the evaporator manufacturer can level out fluctuations in PM costs and provide a better-controlled wastewater disposal system consistent with ISO 9001 and 14001.

Energy Cost of Evaporation

Finally, the energy cost of evaporation (frequently the only cost considered) must be estimated. But here there is room for much more precision in calculating cost. Your firm’s present rate of energy consumption establishes a level of cost-per-unit-energy that, together with the evaporator’s spec sheet, allows a very close estimate of operating cost. That is, of course, assuming the spec sheet "tells all". It is possible to understate energy requirements and thus overstate efficiency. You can conquer that engineering calculation with a little help in understanding evaporator efficiencies.

Those who are contemplating the use of a wastewater evaporator should understand that the energy consumption required to evaporate a given volume of water can vary significantly from one evaporator design to another. It is important to remember that the efficiency of the evaporative equipment (output divided by input) will determine a significant portion of the actual operating cost.

There are several different ways that this information can be presented and applied. The most meaningful is by comparing the amount of fuel delivered to the equipment with the amount of water that is evaporated over a given time.

Laws of Thermodynamics

The basis for these calculations is the set of principles known as the Laws of Thermodynamics. These laws define a British Thermal Unit (BTU) as the unit of heat energy required to raise the temperature of one pound of water by 1°F. The evaporation process requires the input of energy to heat the water to its boiling point of 212°F and additional energy to change this water from a 212°F liquid to a 212°F vapor.

If you wish to evaporate one pound of water (about one pint) at 60°F, the energy requirement is calculated as follows:

Step 1: Raise the water to the boiling point. If you have one pound of water and you add 152 BTU to it, you will raise the temperature from 60°F to 212°F. 1 pound of water 1 BTU/pound (212°F - 60°F) = 152 BTUs °F

Step 2: Add enough energy to change the phase of the water from liquid to vapor. This change is interesting because it takes a much greater amount of energy to accomplish and, rather than increasing the temperature, a physical change from liquid to vapor occurs instead. Engineering tests have determined that it requires 970 BTUs per pound of water to accomplish this phase change. If you have one pound of water at 212°F and you add 970 BTUs to it, it will change from a liquid to a vapor. In other words, it will evaporate.

Therefore, the total energy required to evaporate one pound of 60°F water is 1122 BTUs: 152 BTUs to raise the temperature of the water to 212°F and 970 BTUs to change that one pound of water from a liquid to a vapor.

Notice that about 13.5 percent of the energy (152 of 1122 BTUs) is used to heat the water to its boiling point (called "sensible heat") and 86.5 percent of the energy (970 of 1122 BTUs) is used for evaporation (called "latent heat of vaporization"). Since both the energy to heat the water and the energy required to evaporate it have to be purchased, both should be included in your energy cost calculations. A common mistake in calculating total energy cost is the omission of one of these two vital energy components

Calculating Efficiency

When choosing an evaporator, an important factor to consider is its ability to deliver 1122 BTUs "into the water" for each pound evaporated. Notice the phrase "into the water". Unfortunately, you don’t pay just for the amount of energy that goes into the water; you pay for the energy that you deliver to the unit – only most of which goes into the water. The balance is either used elsewhere or is lost (up the stack, through the walls, etc). Calculating this inefficiency is a little more complicated.

There are typically 1000 BTUs in one cubic foot of natural gas at 60°F; this is what you are buying. Gas distribution companies blend their products to achieve this standard value. You release these 1000 BTUs when you burn this gas. When burned perfectly, under stoichiometric conditions, they liberate only heat, carbon dioxide (CO₂) and water.

This CO₂ and water are as hot as the flame (about 1800°F) at some point during combustion. By transferring this heat into the water (most commonly through a tubular heat exchanger), you heat the water and cool down these combustion gases. This is what heat transfer is all about. The more heat you transfer, the hotter the water and the cooler the combustion gases. If you were able to keep the gases and the water in contact long enough to cool the combustion gases down to 60°F, you would liberate the full 1000 BTUs from each cubic foot of natural gas.

But, if you are boiling water, you can’t cool these gases down below the temperature of the water at boil because it is impossible to heat 212°F water with 60°F gas! So what does this all mean?

It means that some of the energy theoretically available in natural gas (or any other fuel) is not really available. This happens to be approximately 14 percent. This means that while the gross heat content of natural gas is 1000 BTUs per cubic foot (what you pay for), the net heat content, which is what is available to be used, is about 860 BTUs per cubic foot. The difference is typically lost as hot exhaust gas. This is the primary source of inefficiency in virtually all combustion equipment.

A second source of inefficiency is more intuitive and can be called "system loss". This is the more familiar heat loss through the walls of the tank or from the surface of the liquid into the air. It can be minimized by insulation. A well-designed system typically has a system loss of about 5 percent.

One of the important points to keep in mind when calculating the cost of your evaporation system is that energy is bought as "gross heat" and approximately 14 percent of it isn’t available for evaporation. The typical system loss accounts for the loss of an additional five percent of the energy. This leaves a maximum of approximately 81 percent of what you buy to actually work toward heating and evaporating wastewater. Claims of efficiency greater than this, are simply not true. This also explains the Gas Companies’ differentiation between Gross Heating Value (100 percent conversion to heat) and Net Heating Value (stoichiometric conditions with no system losses).

Other Energy Options

Most evaporators sold today use gas (natural or LP) as a fuel; however, evaporators can also be operated using electricity, steam, or oil. Very few units are supplied with fuel oil burners because of the far greater potential to "soot-up" the inside of the heat exchanger, creating an insulating layer and thereby decreasing efficiency dramatically.

The efficiency calculation for electric units is easier than for gas-fired units. The number of BTUs required to evaporate a pound of water remains the same. For electricity, 3413 BTUs are available per kilowatt (KW). The number of BTUs available from a pound of steam is dependent on the steam pressure. Steam tables provide the properties of saturated steam at different pressures. For instance, at 15 psig, about 1164 BTUs are available per pound of steam.

An evaporator manufacturer can supply the required KWs or pounds-per-hour of steam at a specified pressure to provide the evaporation rate of each evaporator model size. Efficiency is calculated by dividing the theoretical energy consumption (1122 BTUs per pound ´ evaporation rate (gallons per hour) ´ 8.33 pounds per gallon) by the actual BTUs per hour required by a specific model.

Bottom Line

Waste treatment and recycling expenses remain one of cleaning's hidden costs. But armed with the right mindset, a cleaning professional can plan for and accurately judge the effect that treatment will have on the bottom line. If evaporation seems like an attractive option, remembering to keep the efficiency of the system and the energy costs in mind when looking for a piece of equipment is key, but is not all one needs. Counting all these cost elements will have a direct impact on one's ability to make wastewater treatment a known part of the complete cleaning equation instead of another hidden cost.

About the Authors
John Baudreau is Director of Sales & Client Support, Herb Elliott is an Application Engineer and Gary Dixon is Marketing Manager for Severn Trent Services – Samsco (Goffstown, NH; 603 668-7111).