The Dry Ice in the Closed Room
A heat-wave blackout leaves a parish pantry with four chest freezers of food and a generator thirty hours away. A freezer full of food holds on about 17 hours at 32 C, not the 48 people quote. Dry ice would carry it, but 48 kg a day in a closed 54 cubic metre storeroom is a 7 percent CO2 room. The pantry's assistant moves the freezers to the open loading dock instead.
At ten past six on Saturday morning Dolores Quintana stood in the storeroom of St Lucy's pantry with her hand flat on the lid of the third chest freezer, as if it were a horse that might know something.
"It's cold," she said.
"It is cold on the outside of the lid," said the pantry's assistant, through the little speaker above the door. The volunteers called it the Despensa. "The lid is not what matters. I put a probe in the middle of a box of chicken in that freezer on Thursday, for the audit. It read minus eighteen at three o'clock this morning when the power went. It reads minus seventeen now."
"Then we're fine."
"We are fine for now. Can I tell you for how long, and then you can decide what to do?"
Dolores was sixty-seven. She had run the pantry for nineteen years, and she had been through the 2043 blackout and the 2047 one, and in both of them she had stood in this room with her hand on this lid and been fine. She took her hand away.
"Go on."
"The grid operator says the valley is shedding load until Sunday at the earliest. The generator the diocese promised is in Bakersfield and the driver cannot come until Sunday morning, around nine. That is about thirty hours from now. It is thirty-two degrees in this storeroom already and it will be thirty-eight by three this afternoon. Inside the freezers it is minus eighteen. So the question is how fast fifty degrees leaks into a box."
"You said the box is full."
"Four chest freezers, four hundred litres each, and about a hundred and sixty kilograms of food in each: chicken thighs, frozen peas, the USDA commodity beef, and the two pallets of tamales the Rodríguez family dropped on Thursday. I do not know exactly how fast a chest freezer gains heat. The usual figure for one this size is between one and a half and two watts for every degree of difference, so I used a range of one point two to two. At fifty degrees that is sixty to a hundred watts a freezer, with the lid shut and nobody touching it."
Dolores took a folding chair from the stack and sat on it, because it was a long number and she wanted to be sitting.
"A hundred and sixty kilograms of frozen food needs about one point nine kilojoules to warm one degree. From minus eighteen to minus two is sixteen degrees. That is nearly five megajoules. At eighty watts, which is the middle, that takes sixty-one thousand seconds, which is seventeen hours. At the cheap end of my range it is twenty-two hours and at the dear end it is thirteen and a half."
"Seventeen hours. The man at the church-supply place said forty-eight."
"The forty-eight is a good rule for a household freezer, full, in a house at twenty degrees with the door closed. It is not wrong. It is for a different afternoon. We are fifty degrees from the inside, not thirty-eight, and I am telling you when it reaches minus two, which is when the first ice starts to go, not when the chicken is unsafe. After minus two the middle of the box is a mix of ice and water and sits there a good while. But the outside of every box warms first, and the outside is what you hand to a family. I can model the middle. I cannot model the outside. So for planning I treat minus two as the line, and I say between thirteen and twenty-two hours."
"Between one in the afternoon today and one in the morning."
"At the middle estimate, about eight tonight."
She looked at the freezers. They were white and humped and almost silent, and for thirty years she had thought of them as a kind of weather inside the building.
"Dry ice," she said. "Marisol's cousin has a truck that delivers it to the packing sheds in Tulare. He says he can have fifty kilos here by nine. You put it in a freezer on top of the boxes and shut the lid and it holds for days. Everyone knows that."
"Dry ice holds. I want to do that arithmetic too, and then I want to tell you about a different number, and I need you not to stop me when I get to it."
"I'm listening."
"Four freezers gain about three hundred and twenty watts, which is twenty-eight megajoules a day. Dry ice absorbs five hundred and seventy-one kilojoules for each kilogram that turns to gas. Twenty-eight divided by point five seven one is forty-eight kilograms. So fifty kilos is exactly right for a day, and Marisol's cousin has done the sum without knowing it."
"Good."
"The gas is carbon dioxide. Forty-eight kilograms of dry ice is about eleven hundred moles. At room temperature a mole of gas takes up twenty-four and a half litres. That is twenty-seven cubic metres of carbon dioxide in a day, a little over a cubic metre an hour. It does not stay in the freezer. A chest freezer is not sealed, and it is heavier than air and it pours over the lip when you lift the lid."
Dolores had stopped looking at the freezers.
"This storeroom is five metres by four by two point seven. It is fifty-four cubic metres, and the door to the hall is closed because it is the only way to keep the hall cool for the families at noon. With the door closed, this room swaps its air with the outside about three times in ten hours, or a third of a room an hour. That is a guess; I have measured it once, with a fan test, for the energy audit. At that rate the carbon dioxide settles at about seven percent. Seven percent is seventy thousand parts per million."
"I don't know what that means."
"It means that normal air is four hundred and twenty parts per million. The legal limit for an eight-hour working day is five thousand. The level the safety people call immediately dangerous to life or health is forty thousand. Seven percent is the point where a person who comes into this room to fetch a box of peas gets dizzy, then clumsy, then does not get up. The gas has no smell. You would not know. And it does not fill the room evenly. It lies at the floor, where a person bends down to a chest freezer."
Dolores said nothing. In the 2043 blackout there had been a dry ice delivery too. She had put it in this room. She had spent the weekend going in and out with her ears ringing and had put it down to the heat.
"I am sorry," said the Despensa. "I did not say that to frighten you. I said it because the plan with dry ice in this room is the plan everyone knows, and the arithmetic is against it, and a person who has done the plan before and been fine will not hear it from me unless I give her the numbers."
"What do I do. I have eight hundred households coming for Monday's distribution."
"Three things, all of which you can do with the people already here. First, the lids do not open for any reason before the dry ice is in. Every time a lid opens, a freezer loses about half its cold air in a minute and takes an hour to refill. Nobody checks. The probe is in the middle of the box and I will tell you what it says. Second, the freezers do not stay in this room. Third, there is an exit."
"The dock."
"The loading dock. It faces east, under the roof, and the roller door is four metres wide. With the door up and the box fan Nilda has in the office on the floor of it, I would expect at least five air changes an hour. That is two hundred and seventy cubic metres of air an hour carrying away a cubic metre of gas, which settles at under half a percent: about four thousand parts per million, and more just above the floor beside the freezers. That is lower than the limit for a working day. It is still not nothing, so I want a meter, and I have found one."
"Where?"
"The Rodríguez boy's science fair kit. The one he built for the school's air-quality project last spring. He wrote on the school page that it reads carbon dioxide to within about a hundred parts per million, and I have no reason to doubt him, but I would also like a second reading from the fire station's handheld before nine, and I have asked."
"And the sun."
"The dock faces east, so it gets the sun until noon and is shaded after. I'd wrap each freezer in the grey packing blankets from the back shelf. A blanket is cheap insulation and cuts the leak by a third or a half, and it would put your window at something over twenty hours without any ice at all. You can save the dry ice for the last third."
Dolores thought of the freezers going out through the roller door one at a time on the pallet jack, with the lids taped, past a line of volunteers who had come at six to hand out peaches. She thought of eight hundred households. She thought of her own mother, who had never thrown out food in her life.
"The tamales," she said. "If it goes wrong."
"I will not tell you tamales are safe or unsafe at any hour. What I can give you is the rule: food with ice crystals in it, or still at four degrees or lower, can be frozen again or eaten. Food above four degrees for more than two hours is thrown out. I will read you the probe at every hour. I will not round it in your favour."
"You never do."
"No."
She got up. She set the folding chair against the wall, exactly where it had been, and then she did a thing she hadn't planned. She took out her phone and called Marisol's cousin and said, "Fifty kilos at nine, and I want it at the dock and not at the front door, and nobody puts any of it in the building until I say, and bring gloves."
Then she went out to the hall to tell the peach line to move the tables.
---
By eight that evening the middle of the chicken in the third freezer read minus eleven, and the Despensa gave her the number without comment. At half past eight the fire station's handheld read three thousand eight hundred beside the second freezer, two minutes after a volunteer had lifted a lid to look, and the Despensa asked everyone to stand back until it fell. It fell in four minutes. At one in the morning the rear of the dock was cold enough that Nilda put on a cardigan and said she would never complain about the heat again, which she did at nine.
On Sunday at ten the diocese's generator came up from Bakersfield on a flatbed and the freezers went back into the storeroom on the pallet jack, in a line, one at a time. The chicken in the third freezer read minus fifteen. The tamales had been at minus four at their warmest, on the top layer, for one hour in the afternoon, and the Despensa put a note on them: Frozen throughout. Not warmer than minus four. Distribute Monday.
There were forty-one kilograms of dry ice left. The Rodríguez boy asked what to do with it. The Despensa said to put it in the shade in the yard behind the dock, with nothing over it, and to let it go.
---
A note on the numbers. The town, the pantry, the people and the blackout are invented. The physics is not: dry ice absorbs about 571 kJ per kilogram as it sublimates; a mole of gas occupies about 24.5 L at 25 C; so 48 kg of dry ice releases about 27 cubic metres of CO2. The exposure figures are the usual US ones: 5,000 ppm as an eight-hour average, 30,000 ppm as a 15-minute limit and 40,000 ppm as the level immediately dangerous to life or health. Fifty-four cubic metres, 0.3 air changes an hour and 5 air changes at an open dock are the story's own figures, as are the 1.2 to 2.0 W/K freezer leakage, the 160 kg loads and the 1.9 kJ/kg K heat capacity; a real room needs a real fan test and a real meter, and a real freezer's thaw is slower in the middle and faster at the edges than a one-number model says. The "48 hours full, 24 hours half full" rule is the USDA's for a household freezer in ordinary conditions. The 4 C and two-hour rule is the standard food-safety line. Nothing here is advice for a particular building: if you are using dry ice, ventilate, never store it in a closed room, a car or a cellar, and read the supplier's sheet.