One of my A/Cs has just failed (Not).

In the Greater Montreal Area (Canada), we have been subject to a prolonged heat-wave, with daily high-temperatures of 35⁰C (+), for approximately a week in a row now. This is expected to continue at least until tomorrow (Thursday, July 5). Luckily, my own home has been protected by two working, 8500BTU air-conditioners until now.

The way an A/C works is such, that it has a compressor-motor, the windings of which are cooled by the return-flow of refrigerant in its gaseous form, after that refrigerant has evaporated in the evaporator, and done its job cooling the home. Yet, these motors are not designed to run with 100% duty, 24/7. They need to cycle off periodically, and one reason for which they normally cycle, is that their function has achieved some sort of (low) target temperature. But, another reason fw the compressor-motor can switch off, is the possibility that its windings themselves, which are linked to yet another temperature-sensor, have overheated.

Even worse than to have the temperature-protection trip once, because of overheated windings, is the very common problem that eventually, the enamel-insulation of the windings may itself fail, causing a permanently defective motor! This tends to happen eventually, because of the cheap way the motors are made.

If that happens, certain turns of the enameled wire, within the motor-windings, will act as if they were the secondary winding of a transformer, to which the still-healthy turns would form the primary winding. A heavy current flows through the short-circuited turns in this way, that can be hard to detect, unless one also measures the exact amount of current drawn by a running motor, and compares it to a known, correct amount of current, which I do not know for the motor in question. But if a winding has in fact started to short in this way, the amount of heat that builds up inside it becomes more acute, of course.

What I am used to from my A/Cs, is that they will run for about 15 minutes, if they fail to reach a target temperature, before their compressors cycle ‘off’. But the A/C in my bedroom, where the temperature is 24⁰C right now, has started to run for only 5 minutes, before turning off. And I have it set to achieve an evaporator-temperature of 20⁰C.

I’ve decided to switch off both my A/Cs temporarily, even though the temperature outside is 35⁰ at the moment, in hopes that they will recuperate. As they are switched off, of course it will start to get warmer in all parts of my home, including in the computer room.

If I should not be able to keep my indoor temperatures under control, I will need to shut down my actual computers next, which are more important to me than the A/Cs, or than my own, personal comfort. In such an event, my blog will also go offline. For the moment, my site and blog are still accessible. But depending on what happens next, there could be some downtime.

(Edit 07/04/2018, 23h05 : )

Apparently, my A/C is still fine. But in order for me to understand this strange behavior, I need to take into account the peculiar way in which my present A/Cs are designed. They are both indoor, portable A/Cs, which have air-ducts that send warm air, with the waste heat, out a window.

Continue reading One of my A/Cs has just failed (Not).

How The Use Of Steam Can Hinder Efficiency.

There exists a concept in Thermodynamics, which describes theoretical limits in the efficiency of all possible heat-engines. This principle states, that if we have a heat-source and a heat-sink, each has an absolute temperature. The ratio between these temperatures defines the highest-possible output of free energy from a heat-engine, as well as the lowest-possible consumption of free energy by a refrigeration-device.

The principle is based on the axiomatic assumption, that there exist no perpetual-motion machines, which simply convert ambient heat into free energy. If we could connect a heat-engine to an air-conditioner, and if these limits could be exceeded, we would have such a perpetual-motion machine.

This also explains why in practice, air-conditioners, refrigerators and heat-pumps can transfer heat from a colder source to a warmer sink, with the energy in heat far-exceeding the electricity consumed. They are all examples in which the ratio of absolute temperatures is close to 1.0 . Actually, what matters is the ratio of the temperatures of the working-fluid in each case, which is actually more oblique than the ratio for air temperatures, because heat-exchangers are never perfectly efficient. And the working-fluids used tend to be similar, because the temperatures at which those systems are designed to work, are also similar.

This also implies that if we wanted to build a heat-engine that uses small temperature-differences to generate electricity, large reserves of heat would be needed as a source, and sent to the sink, before even small amounts of electricity result – which might sometimes be available – but which constrains the system, regardless of what type of heat-engine is used.


Well, in Industrial Power Generation, the temperatures which the heat-source can be run at, depend firstly on what type of fuel is burning, but also depend on the range of temperatures at which water will boil. At 1 atmosphere of pressure, water only boils at 100⁰C, which is also 373K, while the external temperature tends to be around 273-300K .

Actually, by keeping the water boiling at much higher pressures, its boiling-point can also be increased. But it is generally not boosted beyond 200⁰C , which corresponds to about 473K . And so, according to basic principles, no power station based on water and steam, can be more than 50% efficient.

(Edit 05/12/2017 : Additionally, my late father, who was a professional Engineer, used to tell me, that something prevents a steam turbine from being more than 50% efficient. But, this is not a subject I know about, even though it would additionally limit the maximum efficiency of steam-turbine-based power-stations, to approximately 25%. )

In theory, if we could operate our heat-engine at 1000K, and its heat-sink still at 300K, we could achieve efficiencies closer to 70%. Mind you, that that point our heat-source might resemble a lightbulb, more than what we are used to, but this would still obey the rules of Thermodynamics.

My only point being, that the use of water, and its associated boiling-points, is an arbitrary decision. There is no magical reason why we must use it. We could use vaporous sodium if we knew how to work with it safely.

If one breaks out, a sodium-fire is a nasty hazard, much more dangerous in its nature than wood or oil-fires already are.

Dirk

Continue reading How The Use Of Steam Can Hinder Efficiency.