Some products/activities are inherently dangerous (e.g., cigarette smoking, skydiving) and yet some people enjoy them and pay good money to partake of them. In most cases, the more dangerous the activity, the fewer people choose to engage in it. One notable exception is the hot beverage – a potentially hazardous product that is enjoyed by millions.

Numerous studies have shown that customers will reject a cup of coffee or tea when it is served at a temperature that won’t scald them if accidentally spilled. Scalds can occur at temperatures as low as 140°F if the contact time is long enough and temperatures above 150°F will almost certainly cause 1st degree burns or worse if a sufficient quantity contacts the skin. Such details notwithstanding, a large percentage of consumers will reject hot beverages for being “lukewarm” if the temperature doesn’t exceed 155°F. In fact, the average coffee temperature preferred in these studies is approximately 185°F and the average preferred tea temperature is above 200°F.

Furthermore, the chemical process associated with brewing coffee or steeping tea is more effective when the water is as hot as possible. More essential flavors and fragrances are extracted from the beans and leaves when the water is close to boiling. This is why tea kettles “whistle” (to ensure the water has reached a rolling boil before pouring into the teacups) and espresso machines “hiss” (to announce the conversion of pressurized liquid water to steam as it migrates through the coffee grounds).

To be fair, these studies also show that the optimum temperature for a beverage to be “consumed” is considerably below 212°F, but customers invariably realize that if they “receive” the beverage at a temperature that is well above optimum, it will reach its “ideal” temperature within a few minutes. On the other hand, the 2nd Law of Thermodynamics ensures that if a beverage is “received” below its optimum temperature, no amount of time will prevail upon it to absorb heat from its surroundings and approach a more desirable (i.e., hotter) temperature.

The other factor that makes hot beverages a dangerous product that consumers willingly enjoy is their ability to mitigate the risk of drinking them. They can slowly sip or aerate the liquid, thus preventing an injurious amount of heat from being transferred into the mouth tissue, even though the temperature is high.

It is worth noting that the term “safe” should never be construed to mean “without risk”. Most people consider driving a car to be a “safe” activity even though they are fully aware that tens of thousands of motorists die each year in collisions. On the contrary, “safe” simply means that the level of risk is “acceptable” to the population who engages in that activity or uses that product. Consumers believe that drinking coffee is “safe” because the benefits of consuming a great cup outweigh the risk of serious injury if they happen to spill it.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2016)

Restaurants are relatively frequent victims of accidental fires. Each year, nearly 1 in 100 eating and drinking establishments experiences a fire loss and 57 percent of those fires are caused by cooking equipment (see http://www.nfpa.org/~/media/files/research/nfpa-reports/occupancies/oseating.pdf?la=en). However, the primary factor in determining fire extent and fire damage is often the status of the exhaust system – how well it was designed and installed, and how clean it has been kept.

NFPA 96 “Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations” (http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards?mode=code&code=96) enumerates dozens of design and maintenance requirements whose intent are to reduce the likelihood of fire initiation and reduce the extent and magnitude of fire damage.

Grease (from cooking operations) is a combustible semi-solid and will accumulate on exhaust system surfaces, even if a well-designed hood and grease removal apparatus is in place. Exhaust duct cleaning is required by NFPA 96 at a frequency that depends on cooking volume. Also, a new standard has been published by the International Kitchen Exhaust Cleaning Association (ANSI/IKECA C-10 “Standard for Cleaning of Commercial Kitchen Exhaust Systems”, see http://www.ikeca.org/news/ansiikeca-c-10-standard) that focuses on inspections, cleaning technologies, and client reporting. Together, these standards provide an authoritative framework for avoiding fires, and for minimizing damage if one should occur accidentally.

One of the requirements in NFPA 96 is “Cooking equipment shall not be operated while its fire‐extinguishing system or exhaust system is nonoperational or otherwise impaired.” Impairment of the exhaust system can arise from any of several maintenance lapses or installation defects (e.g., exhaust fan failure, plugging of exhaust system passageways by accumulated grease, and breaches in the exhaust ductwork where air can infiltrate).

This author investigated a very large fire loss that originated in the basement kitchen of a 10-story hotel, and exhaust system impairment was a major factor. Through interviews, inspections, and analyses, the investigation team discovered the following:
• The kitchen staff had complained of smokiness and uncomfortably high temperatures in the cooking area a few weeks before the fire.
• The fan was determined to be working properly.
• The grease extraction system (type: “water wash”) was heavily laden with grease, and likely had never been serviced by the hood manufacturer since it was installed nearly 20 years prior to the fire. (NFPA 96 requires the owner to engage a trained and certified technician for mechanical system maintenance. This requirement typically excludes the cleaning contractor.)
• The cleaning contractor couldn’t access the internals of the water wash system and several other duct spaces, so they only cleaned the accessible areas of the exhaust system.
• The cleaning contractor was tasked by restaurant management to perform a special cleaning about a week before the fire, ostensibly to find and remove the grease obstruction. Upon arrival, the contractor found one cleanout hatch wide open, and one-half of the two-piece hatch cover assembly missing. Per his custom, he only cleaned accessible areas, and tried to close the cleanout opening with the compromised hatch cover. He reported the missing hatch cover piece to management but was unaware of the major grease accumulation that remained inside the water-wash hood.
• After the fire, at one of the evidence inspections, the investigation team discovered the second half of the hatch cover assembly inside the water-wash hood, directly below the cleanout port where it should have been installed.

Based on these facts, this author concluded that on the day of the fire (and probably each of the other days since the special cleaning had been performed one week before), the cooking staff had removed the remaining half-cover of the cleanout in order to improve their working conditions in the kitchen. We reached this conclusion in spite of the fact that none of the chefs admitted doing so.

We developed a numerical model to determine how badly the open cleanout port would “impair” the exhaust system, and estimated that with the cleanout port wide open, the flow through the water-wash hood would drop by almost 40% and the flow into the open cleanout port would exceed the hood flow by more than 25%. This would have had the desired effect of cooling the staff’s working environment, while simultaneously compromising the effectiveness of the grease removal device and permitting grease to enter the exhaust duct unabated.

Most importantly, the burn patterns (and some witness statements) indicated the fire actually originated in the duct near the open cleanout port (i.e., downstream of the water-wash hood) and the most likely cause of ignition was a cooking ember that was sucked into the opening and landed on accumulated grease, igniting it.

The primary lessons from this incident were: (a) the cleaning contractor should have explicitly informed the restaurant that they were unable to clean the water wash hood internals; (b) the restaurant shouldn’t have expected the cleaning contractor to maintain and clean the water-wash hood, a complicated piece of machinery that they weren’t trained to service; (c) the cooking staff shouldn’t have compromised the exhaust system by opening the cleanout port, because doing so permitted the fuel (extra grease) and the ignition source (the ember) to meet in that location and initiate the fire.