Slayer: Why the Intersection of Brewing & Steaming Matters!
Oct 10, 2009
(Slayer’s 1.4 gallon dedicated preheat tank supplies buffered 193 degree F water to brew & steam tanks)
In newer generation espresso machines, brew temperature stability is achieved through three features: independent, but relatively small brew tanks, PID for temperature control, and a supply of pre-heated water to each brew group.
On most machines with independent brew tanks the main water inlet to the espresso machine comes through the steam tank, and there is no cold water inlet to brewing.
On these machines, the steam tank supplies pre-heated water to each brew group through a heat exchanger (HX) that draws heat from the steam boiler and distributes it to where it’s needed. This happens in real time as you pull shots. During peak periods this hot water draw can be as high as 12-16 oz per minute.
Preheated Water Buffering
Slayer approaches preheating completely differently. The steam and brew systems in a Slayer operate independently of each other, so Slayer forgoes the use of an HX to bring heated water from the steam boiler to the brew tanks. Instead, every Slayer has an independent tank dedicated to buffering cold incoming water. Incoming water is heated to 193 degrees F (though this is adjustable) and held in a reserve of up to 1.4 gallons. This water is supplied to each brew tank on demand. The steam tank is also supplied with pre-heated water from this source, enhancing performance in this regard.
What does this do?
We have found under heavy-load testing that an HX-approach reduces the capacity of the steam boiler to meet peak needs. Intuitively, it is easy to see why. If incoming cold water must be heated by the steam system this energy is no longer available for steaming, and there is a corresponding reduction in capacity–even with the addition of a mix valve.
In HX systems like this it is even possible to experience brew temperature creep as hotter water from the steam tank displaces the PID conditioned water of the brew tanks.
The practical implications of this difference were recently posted when we videoed Slayer under full steam. This video was stopped after 45 minutes because there was simply no limitation to Slayer’s steam capacity and viewers would have been watching for days or until the power was cut off to the machine itself.
Independent systems through-out
Since for all practical purposes there is complete separation of the brew and steam functions on Slayer, continuously brewing or steaming has no detrimental effect of either system.
In developing Slayer, we used a custom wattage density calculator for process systems from the Physics Department at University of Washington to determine the ideal wattage and capacity values for Slayer’s tanks and elements.
In our calculations, heat dispersion variables included values for shots being pulled at temperatures in a range (195-207 F), at discreet volumes of two ounces, at a rate reflecting peak cafe through-put. These calculations also included variables for heat dispersion at the surfaces of the materials in the system, tanks, tubing, groups etc. which accounted for material type, tank wall thicknesses, boiler shape, as well as energy loss through steaming.
This effort resulted in thermal optimization under all operating conditions. You don’t run out of steam and you can’t screw up your brewing parameters because of work load.
Recently Slayer has been approached by high volume cafe operators who have said they are experiencing real world steam deficiencies when their machines are called upon to pick up from a jog to a sprint during peak periods. Some of these operators have claimed this is the reason they’ve installed two machines instead of one.
An extra machine on the counter is often rationalized as a “back-up”, in case one machine goes down. In the past even Starbucks eventually began installing four-group machines, not for the extra brewing capacity, but because their steam requirements stretched the capacity of smaller machines.
But an extra machine is a significant expense. It should be unnecessary to add a second machine to meet the needs for steam for the volume of coffee you are able to brew on a single unit. That is, if you can brew the shots on your machine, that machine should be able to supply your steamed milk needs too–without compromising your brew parameters.
Eric in Seattle