Engineering Units, Part 2
In my last column, I defined and manipulated some of the common thermal and energy units. Before I leave that topic, I want to clarify an antique engineering unit, the ton. It is still used in the refrigeration and air conditioning industry, but in this context, it is not a unit of mass. It expresses a rate of removal of heat, which makes it a unit of power, negative though it is. This brand of ton equals 3.5 kW or 12,000 BTU/hr.
I'll now turn to totting up the energy requirements of a heat process and the challenge of comparing energy prices.
Say you are designing or constructing a new heating process. Before you start calculating, pause a while. There is no better guide than your practical experience and design and performance records of a similar process. It could be one that you have operated or designed. If there isn't a close match in your plant, it may be well worth the trouble to locate a similar process and drive out to examine it.
In any case, you would still like to know where the heat input goes. First, you need the mass, specific heat and temperature rise of all parts of the process. Same for the work and any heat transfer medium. Second, you need the various heat losses at working temperature. These include convection (for example, flue gases and air throughput) as well as surface radiation and convection to the environment. Losses can be hard to calculate, so this is where you apply testing and experience.
Now you can determine both the kWh input needed to reach working temperature and the kW to maintain it. Next, ensure that you have the power to reach temperature fast enough to maintain the production rate you want. However, some loads demand a slow, controlled heatup rate and a defined hold time to avoid thermal shock to the process and work and to allow heat to soak thoroughly into the work. Under these kinds of restraints, there isn't much you can do to speed production, but there is a possible good side effect. However, if you are using electric heating, your electric metering could record a restricted peak startup power and save money on the demand charge component of your bill. You will not necessarily use less energy per batch.
Paying for Your EnergySo, you have worked out your energy requirements and made a provisional choice of heat source. You may change that choice when you see the heating costs. Be warned, comparing energy costs is not easy. First, vendors cannot agree on a common unit for selling energy. Second, the degree of concealment and convolution in billing will likely defeat your easy choice of best buy. The pricing formula varies with fuel, vendor and location.
Oil is sold by the liter or gallon, and delivery may be a separate item. You need to know the calorific value (CV), typically given in BTU per gallon.
Natural gas sells by the cubic foot or cubic meter, and the CV could be given in BTU/ft3 or MJ/m3. You may not find the CV of either oil or gas on your bill or quotation. Again, delivery could be a separate item.
Electrical energy sells by the kWh, perhaps with an off-peak discount, plus a possible monthly charge per kVA of maximum demand. Sometimes the advertised cost per kWh omits the transmission charge and sales tax, but these are rolled in to your monthly bill with no breakdown shown. These roll-ins can double the posted price.