A modest evaporator consists of an arch, a flue pan, a syrup pan, and a smokestack.
So you love making maple syrup. Your friends and family love that you make maple syrup. You realize that you need to make more syrup to satisfy the requests for your delicious maple syrup. If this is the case, you may want to think about investing in an evaporator, which is simply a cooking pan or a set of cooking pans placed over an arch (sometimes referred to as a firebox).
There are three reasons to think about cooking on an evaporator: You’re tapping more trees, you need to cook faster, or you would like to cook more efficiently. If you can’t invest any additional time in cooking sap, the quality of your syrup is suffering, or the syrup comes out very dark because it is over heat for too long, cooking on an evaporator may remedy these issues.
An evaporator basically comprises stainless steel rectangular pans over a heat source called an arch. Most arches are built to accommodate wood or fuel oil, but they can also be heated with natural gas, propane, or high-pressure steam. Dry hardwood works best, as it burns very hot. Evaporator add-ons, such as reverse-osmosis machines and preheaters (see Additional Components, page 61), will reduce the amount of fuel used during a syrup season.
There are two basic types of evaporators: flat-pan evaporators and flue-pan evaporators. Flat-pan evaporators utilize a pan or pans that have flat bottoms. Flue-pan evaporators use a pan that has corrugated flues above or below the bottom of the pan.
To make sure you have enough fuel to finish all your sap, use the following as a guide:
Flat-pan evaporators are generally used by hobbyists who are cooking for fewer than 150 taps. Flat pans come with or without dividers; dividers allow the sap to flow through the pan in channels. Whether you cook in a pan with or without dividers, it is difficult to completely finish your syrup on a flat-pan evaporator. Most syrup makers who use flat-pan evaporators cook their sap until it is almost finished, then move the “thin” syrup into a smaller pan and finish it over a heat source that is easy to control, such as a propane burner.
Flat-pan evaporators are generally less expensive than flue-pan evaporators, but they still cook sap quite efficiently. Many manufacturers make such units, and they can be purchased from a maple syrup supply dealer. Ask a dealer for assistance when determining which style and size is right for your operation.
Flue-pan evaporators include two pans — one sitting on top of the back half of the arch (called the flue pan) and one sitting on top of the front half of the arch (called the syrup pan). The back of the evaporator is where the smokestack comes out of the arch. The wood or fuel goes into the arch at the front of the evaporator.
Sap flows continuously from the point of entry, where sap comes into the pan, to the point of draw off, where finished syrup leaves the pan. Cooler and less dense sap stays toward the back as the hotter and denser syrup travels to the front. As water evaporates from the sap, the sugar content increases, and the sap flows through the evaporator, until it is drawn off the front of the syrup pan as pure maple syrup.
A modest evaporator consists of an arch, a flue pan, a syrup pan, and a smokestack.
Raised or dropped flue pans. The syrup pan is flat and has dividers. The back flue pan has either raised or dropped corrugated channels running through it. As sap enters the flue pan from a holding tank, a valve on the tank is open, and sap slowly and continuously drizzles into the pan. The channels running though the flue pan add surface area to the pan and will thus decrease cooking time.
In a raised flue pan, the channels are flush with the bottom of the pan and rise up into the pan. In a drop flue pan, the channels drop down below the bottom of the pan into the arch. A raised flue has two float boxes, and a drop flue has one float box; this is a regulating tool that controls the level of sap in your pan, keeping the level consistent during cooking. Floats utilize a valve system to regulate this level automatically. A float box consists of the box, the float, and the mechanism that opens and closes a valve.
Because it has two float boxes, a raised-flue evaporator allows the sap in your flue pan and the sap in your syrup pan to be at two different levels. Because of the difference in levels, sap can run more quickly from the flue pan into the syrup pan if necessary. This may come in handy if the level in your syrup pan is suddenly so low that it is in danger of going dry and burning. However, because it has two float boxes rather than one, a raised-flue evaporator has more moving parts that could potentially need replacing.
Functionally, those are the main differences between a raised- and a dropped-flue pan. The two pan types, if equal in size and number of channels, should cook off the same amount of sap per hour.
The flues in a raised flue pan rise up above the evaporator’s arch. The flues in a drop flue pan drop down into the evaporator’s arch.
You can purchase a number of components to customize your evaporator. Preheaters, steam hoods, and reverse-osmosis machines are just a few.
Evaporator add-ons can appear to be costly, but if you need to decrease your cooking time, they can be well worth the additional cost. A maple syrup supply dealer can help you calculate the long-term cost savings.
Most evaporators are special ordered well before the syrup season begins. You need to do some setup work to prepare for an evaporator, which you may not want to do in the snow and the cold. To avoid this, purchase your evaporator several months prior to syrup season.
Larger manufacturers begin building evaporators in the late spring and early summer. This is a good time to decide what you want and to place an order. Purchasing an evaporator during the summer months will ensure that you’ll get exactly what you want. You’ll receive your evaporator well before syrup season, and you’ll have ample time to set it up before the sap starts to run. However, if you’re fairly flexible and intend to buy a relatively basic model, a dealer near you may have something in stock that will suit your needs.
The longer sap is exposed to heat, the darker in color and stronger in flavor it gets, possibly taking the syrup from one grade to the next. An evaporator decreases the time that sap is exposed to heat because you can continually draw finished syrup off the syrup pan. Therefore an evaporator can potentially make better-quality syrup than a flat pan. It is possible to make nice light syrup on a flat pan; it is just easier to do it with an evaporator.
When purchasing an evaporator, plan for a middle-of-the-road sap run, not the best- or worst-case scenarios. This will ensure that your evaporator is not under- or oversize for your production.
Evaporators come in various sizes. The best size for you will depend on a number of factors:
With a gravity system. On an average day, if you are collecting by gravity (either in pails or a gravity-flow tubing system), you can plan for 1 gallon of sap per tap. So if you have 100 taps and 10 hours to complete your cooking, you will need an evaporator that will boil off 10 gallons of sap per hour.
With vacuum tubing. If you are collecting with a vacuum tubing system, you can plan for an average of 1.5 gallons of sap per tap per day. Again, if you have 100 taps and 10 hours to complete your cooking, you will need an evaporator that will boil off 15 gallons of sap per hour.
The evaporation rate is about 1 gallon per square foot of surface area. So to cook off 10 gallons of sap per hour, you’ll need a pan with 10 square feet of surface area. A 2-foot-by-5-foot or a 3-foot-by-3.5-foot flat pan will give you approximately the right square footage. In a flue pan, the channels will add surface area, giving a smaller pan a lot more square footage.
To maintain this cooking rate, you’ll also need to maintain a good hot fire underneath your pan. For instance, it’s a good practice to fire very consistently, every 10 to 15 minutes, with very dry hardwood. Some producers set a timer so they can maintain a consistent firing schedule.
Consult a reputable maple syrup equipment dealer for advice on the appropriate size pan or evaporator for your operation.
The arch of an evaporator needs to be insulated for maximum efficiency. When cooking with wood, the arch is generally insulated with firebrick. When cooking with oil or gas, the arch is insulated with a ceramic blanket or firebrick.
Oil- or gas-fired arches often come insulated, but wood-fired arches do not. Once firebrick is installed, the evaporator becomes extremely heavy. The evaporator would be very difficult to move if it were bricked prior to putting it in place. When purchasing an evaporator, make sure your dealer gives you bricking instructions, so you can install the brick once you have placed your evaporator in the desired location.
It is best to place your evaporator on a concrete floor. If this is not possible or practical, you may place it on patio blocks or a level layer of gravel. The foundation for your evaporator needs to be level and must remain so as long as your evaporator sits there. You should not place an evaporator on bare ground. The freeze-thaw cycle will cause the evaporator to sink into the soil and become unlevel. If your evaporator is not level, the sap will not flow evenly through the pans. If the sap is uneven in the pans, a shallow spot could develop during cooking, and the pan could scorch or burn.
In a wood-fired evaporator, the arch is almost always open on the bottom, under the grates. Hot ash and embers will fall through to the ground. You certainly don’t want to install your evaporator on a wood floor!
Once your evaporator is in position, make sure the smokestack lines up with the hole in your roof before installing the firebrick. That way, you can adjust the evaporator’s position before it is too heavy to move easily. If your arch is wood fired, your smokestack should be twice as tall as your pans are long. If your arch is oil fired, your stack can be shorter. With either type of arch, your stack must clear the peak of the roof of your cookhouse by 3 feet to maintain a good draft.