It’s hard to notice the lack of something, but there’s been a sharp drop-off in the the number of serial killers. Nearly gone are folks like John Wayne Gacy (the clown killer), Jeffrey Dahmer (severed heads, cannibalism, necrophilia), Gary Ridgway, “The Green River Killer” (71 prostitutes killed). Mostly, they were sexual sadists, men who’d have sex with strangers (able or female) and then kill them. In 1987, there were 198 active in the US and many more inactive; by 2018 it was down to 12. And these few are less-prolific, and less-colorful, like Anthony Robinson, “the shopping cart killer”, who killed 4-6 in DC, transporting the bodies in shopping carts.
It’s not clear why there are so few these days. Perhaps it’s the prevalence of surveillance cameras, or improvements in DNA and other pic technology. But these explanations don’t explain why there were so few before 1960. There were some mass murderers, “Jack the Ripper,” “the Boston Strangler” but few before 1960.
Police like to credit the drop off to their detective skills, but there are still plenty of violent crimes that go unsolved, about half the murders in Detroit for example, or most of the rapes in Europe. I suspect that serial killing spiked up in the 60s because of a spike in friendliness, and spiked down in the 2000s because it ended. Before the hippy era, people were cautious of gangsters, rapists, homosexuals, and spies under the bed. But that changed in the 60s. Folks thought it was cool to hitchhike, or pick up random guys. Now, we’re back to being cautious.
A personal story: I was visiting Toronto in the late 1980s and someone I didn’t know overheard that I was planning to drive back to Detroit that afternoon. He asked if I would not mind driving his teenaged daughters to Detroit to see their grandmother, and I said “yes.” At the border, the guards asked who these girls were, and I said I didn’t know. I hadn’t asked. The border guards let us through without passports after a call to the grandmother. I would not be as ready to offer a ride today, and the parents would not be as trusting, nor would the guards.
Serial killings are down since 1990, but mass shootings are up.
Despite much the stricter gun laws, there’s been a rise in crime and a steady growth in the number of murderers in our major cities. There’s also been a rise in synagogue attacks, and a rise in mass murders. These folks kill many in one day or as part of gang-drug activity. Stricter gun laws seem to have made things worse, not better. They do not stop the killers and they hamper the defenders. I took a look at synagogue attacks, and find a pistol would have helped.
On a societal level, I think it would help to have fewer illegal aliens, or aliens who enter with no positive record or skills. It would help to have psychological treatment and lockup for crazy folks and prisoners. Currently, we send violent crazy folks out on the streets until they do something true horrific. More consistent prison sentences are needed for criminals too. We’ now’ve come to use the courts for political theater: Biden’s son should not go to jail for years because he lied on a gun purchase, nor should Trump get for putting down a prostitute as “legal fees”. Nor is his half-billion dollar fine appropriate. Minor crime deserves minor punishment. As a result of our crazy courts, violent criminals are let go as with the MSU killings near me. He was a crazy violent black man, and there was no way, in the law to give him a short sentence, or counseling, or job training. When our incarcerated leave prison, they have anger, plus no jobs or skills. Don’t be surprised when these folks turn to violent crime.
Germany’s green transition is a disaster. Twenty years ago, Germany had 23 nuclear power plants that generated 30% of the country’s electricity cleanly, cheaply, and reliably. These plants have all been shut by the government as part of a commitment to clean energy. What could be cleaner? Germany has switched to a mix of wind and solar, plus a significant shift to coal power. Wind and solar use a lot of land compared to nuclear, and they break down leaving fields of debris. There is now a lack of electricity to power homes and industries, and what power there is, is unreliable, due to the many dark windless days in Germany.
The lack of reliable electricity is crippling German industry now that Russian gas has been cut off. In this environment, why would the Germans order special trains and boats that burn, hydrogen that’s made from electricity and natural gas? My understanding of the reason is that, Germany sometimes has too much wind power and nothing to do with it. They plan to store this excess by making hydrogen that they can use to power their trains and boats. The cost is high, and the efficiency is poor, but the electricity is free.
Hydrogen is not as compact a fuel as gasoline, nor is it as cheap as electricity, but it’s cleaner than gas, and in some ways it’s better than battery-stored electricity. While hydrogen takes a lot of storage space relative to gasoline, high pressure helps, and the storage is cheaper than with batteries. Also, hydrogen fuel is transferred faster than electric fuels. Trains and ships are chosen for hydrogen because they are good at carrying bulky items. The transition to hydrogen is relatively straightforward with trains, since many are already powered by electricity. Hydrogen fuel cells can make the electricity on board (in theory), while avoiding the need for expensive overhead wires. The idea sort-of makes sense.
Germany’s first hydrogen train. cancelled after 1 year of poor operating.
The first German train to use hydrogen powered them with fuel cells that generated electricity. It began service in October 2022, but the fuel cells proved unreliable. Service ended one year later, October 2023, replaced by polluting diesel (see here). The Hannover line plans to replace these with battery-powered trains over the next few years. There are also plans for a hydrogen-powered ferry, but it is not clear why the ferry should prove more reliable than the train, or cheaper.
San Francisco’s hydrogen-powered ferry, $30 million, 15 knots top speed, 75 passengers, no cars. Long delayed.
In the US, the Biden administration has paid, so far, $30 million for a hydrogen ferry in San Francisco. It’s two years behind schedule and over cost, taking only 75 passengers and no cars at 15 knots, 17mph. In the US, and likely in Germany, most of the hydrogen will be made from natural gas. A better solution, I think would be to power the ferris and trains by natural gas and to store the excess electricity in land-based batteries or as land-based hydrogen for land-based fuel cells.
Germany is committed to electric trains, though, and hydrogen provides a route to power these trains with excess electricity. German customers take the train, in part, because they like them, and in part because German politicians have banned short-hop planes on competing routes, and subsidized electric trains. Yet another option to balance times of excess solar and wind power would be to subsidize electric cars, or at least allow theirs owners to trade electricity: to buy electricity when it’s cheap and resell it to the grid when demand and prices are high.
Heat exchange is a key part of most chemical process designs. Heat exchangers save money because they’re generally cheaper than heaters and the continuing cost of fuel or electricity to run the heaters. They also usually provide free, fast cooling for the product; often the product is made hot, and needs to be cooled. Hot products are usually undesirable. Free, fast cooling is good.
So how do you design a heat exchanger? A common design is to weld the right amount of tubes inside a shell, so it looks like the drawing below. The the hot fluid might be made to go through the tubes, and the cold in the shell, as shown, or the hot can flow through the shell. In either case, the flows are usually in the opposite direction so there is a hot end and a cold end as shown. In this essay, I’d like to discuss how I design our counter current heat exchangers beginning a common case (for us) where the two flows have the same thermal inertia, e.g. the same mass flow rates and the same heat capacities. That’s the situation with our hydrogen purifiers: impure hydrogen goes in cold, and is heated to 400°C for purification. Virtually all of this hot hydrogen exits the purifier in the “pure out” stream and needs to be cooled to room temperature or nearly.
Typical shell and tube heat exchanger design, Black Hills inc.
For our typical designs the hot flows in one direction, and an equal cold flow is opposite, I will show the temperature difference is constant all along the heat exchanger. As a first pass rule of thumb, I design so that this constant temperature difference is 30°C. That is ∆THX =~ 30°C at every point along the heat exchanger. More specifically, in our Mr Hydrogen® purifiers, the impure, feed hydrogen enters at 20°C typically, and is heated by the heat exchanger to 370°C. That is 30°C cooler than the final process temperature. The hydrogen must be heated this last 30°C with electricity. After purification, the hot, pure hydrogen, at 400°C, enters the heat exchanger leaving at 30°C above the input temperature, that is at 50°C. It’s hot, but not scalding. The last 30°C of cooling is done with air blown by a fan.
The power demand of the external heat source, the electric heater, is calculated as: Wheater = flow (mols/second)*heat capacity (J/°C – mol)* (∆Theater= ∆THX = 30°C).
The smaller the value of ∆THX, the less electric draw you need for steady state operation, but the more you have to pay for the heat exchanger. For small flows, I often use a higher value of ∆THX = 30°C, and for large flows smaller, but 30°C is a good place to start.
Now to size the heat exchanger. Because the flow rate of hot fluid (purified hydrogen) is virtually the same as for cold fluid (impure hydrogen), the heat capacity per mol of product coming out is the same as for mol of feed going in. Since enthalpy change equals heat capacity time temperature change, ∆H= Cp∆T, with effectiveCp the same for both fluids, and any rise in H in the cool fluid coming at the hot fluid, we can draw a temperature vs enthalpy diagram that will look like this:
The heat exchanger heats the feed from 20°C to 370°C. ∆T = 350°C. It also cools the product 350°C, that is from 400 to 50°C. In each case the enthalpy exchanged per mol of feed (or product is ∆H= Cp*∆T = 7*350 =2450 calories.
Since most heaters work in Watts, not calories, at some point it’s worthwhile to switch to Watts. 1 Cal = 4.174 J, 1 Cal/sec = 4.174 W. I tend to do calculations in mixed units (English and SI) because the heat capacity per mole of most things are simple numbers in English units. Cp (water) for example = 1 cal/g = 18 cal/mol. Cp (hydrogen) = 7 cal/mol. In SI units, the heat rate, WHX, is:
The flow rate in mols per second is the flow rate in slpm divided by 22.4 x 60. Since the driving force for transfer is 30°C, the area of the heat exchanger is WHX times the resistance divided by ∆THX:
A = WHX * R / 30°C.
Here, R is the average resistance to heat transfer, m2*∆T/Watt. It equals the sum of all the resistances, essentially the sum of the resistance of the steel of the heat exchanger plus that of the two gas phases:
R= δm/km + h1+ h2
Here, δm is the thickness of the metal, km is the thermal conductivity of the metal, and h1 and h2 are the gas-phase heat transfer parameters in the feed and product flow respectively. You can often estimate these as δ1/k1 and δ2/k2 respectively, with k1 and k2 as the thermal conductivity of the feed and product, both hydrogen in my case. As for, δ, the effective gas-layer thickness, I generally estimate this as 1/3 the thickness of the flow channel, for example:
h1 = δ1/k1 = 1/3 D1/k1.
Because δ is smaller the smaller the diameter of the tubes, h is smaller too. Also small tubes tend to be cheaper than big ones, and more compact. I thus prefer to use small diameter tubes and small diameter gaps. in my heat exchangers, the tubes are often 1/4″ or bigger, but the gap sizes are targeted to 1/8″ or less. If the gap size gets too low, you get excessive pressure drops and non-uniform flow, so you have to check that the pressure drop isn’t too large. I tend to stick to normal tube sizes, and tweak the design a few times within those parameters, considering customer needs. Only after the numbers look good to my aesthetics, do I make the product. Aesthetics plays a role here: you have to have a sense of what a well-designed exchanger should look like.
The above calculations are fine for the simple case where ∆THX is constant. But what happens if it is not. Let’s say the feed is impure, so some hot product has to be vented, leaving les hot fluid in the heat exchanger than feed. I show this in the plot at right for the case of 14% impurities. Sine there is no phase change, the lines are still straight, but they are no longer parallel. Because more thermal mass enters than leaves, the hot gas is cooled completely, that is to 50°C, 30°C above room temperature, but the cool gas is heated at only 7/8 the rate that the hot gas is cooled. The hot gas gives off 2450 cal as before, but this is now only enough to heat the cold fluid by 2450/8 = 306.5°. The cool gas thus leave the heat exchanger at 20°C+ 306.8° = 326.5°C.
The simple way to size the heat exchanger now is to use an average value for ∆THX. In the diagram, ∆THX is seen to vary between 30°C at the entrance and and 97.5°C at the exit. As a conservative average, I’ll assume that ∆THX = 40°C, though 50 to 60°C might be more accurate. This results in a small heat exchanger design that’s 3/4 the size of before, and is still overdesigned by 25%. There is no great down-side to this overdesign. With over-design, the hot fluid leaves at a lower ∆THX, that is, at a temperature below 50°C. The cold fluid will be heated to a bit more than to the 326.5°C predicted, perhaps to 330°C. We save more energy, and waste a bit on materials cost. There is a “correct approach”, of course, and it involves the use of calculous. A = ∫dA = ∫R/∆THX dWHX using an analytic function for ∆THX as a function of WHX. Calculating this way takes lots of time for little benefit. My time is worth more than a few ounces of metal.
The only times that I do the correct analysis is with flame boilers, with major mismatches between the hot and cold flows, or when the government requires calculations. Otherwise, I make an H Vs T diagram and account for the fact that ∆T varies with H is by averaging. I doubt most people do any more than that. It’s not like ∆THX = 30°C is etched in stone somewhere, either, it’s a rule of thumb, nothing more. It’s there to make your life easier, not to be worshiped.
