Steam Blanchers
VOL. 19 NO. 6
They are simple, low priced and unfortunately, pretty much out of favor. For many applications, atmospheric steam blanchers are a sensible approach. This type of blancher consists of a belt carrying product through a steam chest. Steam distribution pipes are located underneath the belt. In order to get product in and out, the ends are relatively open requiring good design to avoid excessive loss of steam. Once the blancher is sized, (a function of product depth, blanch time and product density) common sense, utilizing the knowledge that heat rises and steam will pass through a hole, will produce an effective design. For most efficient operation, the sides and the ends of the steam chest must be sealed to the belt, so the steam can only leave the area underneath the belt by passing up through the product. The steam distribution pipes should furnish steam throughout the steam chest. The design should include provision for a higher volume of steam at the infeed end to heat incoming product relatively quickly. Heat is transferred to the product as the steam condenses. Temperature is determined by the use of sensors located underneath the belt, where they are exposed to hot condensate dripping off the product. These sensors provide temperature information to the controller, which determines how far the steam supply valve opens.
Good atmospheric steam blancher operation requires total coverage of the belt by- product, which should be no deeper than steam will penetrate. Operating temperature must be below the boiling point of water at the blancher's location. If the temperature set point is too close to, or above the boiling point of water, too much steam will be furnished and the excess will escape out the ends of the blancher. For products that are hard to penetrate, steam distribution pipes can be installed above the product belt to increase the depth of product that can be blanched. This approach, while normally effective, has two potential disadvantages. Because steam distributed from overhead does not have to pass through the product, vapor is more apt to escape from the ends of the blancher. Since the distribution pipes are above the product and heat rises, the steam emitting from the pipes must be aimed at the product. Unfortunately, steam leaving the distribution pipes is considerably hotter than the blancher operating temperature. This can cause overcooking of small amounts of product at the top of the bed. This problem is avoided underneath the belt by aiming the steam pipe openings down, so the steam can cool before hitting the product.
Too often atmospheric steam blanchers have not been properly designed and rationally operated, resulting in excessive amounts of steam escaping. This has led to the use of rotary steam locks and hydrostatic end seals to contain the steam at the ends of the blancher. These methods are effective and in some cases have added advantages. The advantages and shortcomings of these systems will be discussed in a future Gem Memo. Still there are many applications, especially seasonal, where an atmospheric steam blancher properly designed and intelligently operated will achieve a lower total ten year cost of operation and ownership than its more complicated alternatives.
VOL. 19 NO.7
When viewed from the perspective of heating product and steam consumption, there are three types of steam blanchers, atmospheric, fan assisted and sealed ends. The simplest, atmospheric, consists of a belt traveling through a steam chest with open ends to accommodate entry and discharge of product. Fan assisted drives the steam vapor into the product making it possible to obtain a uniform blanch with deeper product beds. Two types of end seals are commonly used on steam blanchers, rotary lock and hydrostatic.
Rotary locks, which are the full width of the blancher belt, consist of a paddle wheel, rotating in a housing. Usually, rubber or UHMW scrapers are utilized at the edge of each paddles to ensure a good seal with the housing. An alternative method of sealing is to machine the housing and edges of the paddle wheel to obtain a very small clearance between the paddle wheel blades and housing. This type of construction, normally used for applications with higher differential pressures, is too expensive for most blanching applications. If the rotary lock is properly designed, there should be no steam leakage when the wheel is stationary. However, to get product in and out of the blancher, paddle wheels have to rotate. On the infeed end rotary lock, product enters the blancher on the side rotating down and a small amount of steam vapor escapes the blancher on the side rotating up. Because heat rises and the outgoing segment of the paddle wheel is partially filled with product, less steam escapes from the discharge end rotary lock.
Hydrostatic end seals use water to block the ends of the steam chest. The most common system utilizes a tank of water at each end. The blancher tail shaft is located at the bottom infeed end of the infeed tank. The belt inclines up into the steam chest. Incoming product is dropped into the water. A combination of water sprays and a recirculation pump sets up movement of water, propelling the product to and spreading it on the belt. In the discharge end tank, pump and sprays propel product toward a separate discharge conveyor. Often this discharge conveyor is the infeed end of a hydrocooler. For some products the infeed tank functions as a rinse or wash tank. This configuration can provide for heat recovery. Hot product warms the water in the discharge tank. Warm water is pumped to the infeed tank to preheat incoming product. One broccoli blancher with two stages of water heating on the discharge end and two stages of product heating on the infeed end, was able to blanch nine pounds of product with each pound of steam.
A more economical discharge end water seal consists of a flume feeding a "U" shaped section of pipe which fills with water trapping the steam vapor. The top of the discharge flume is sealed to the discharge opening of the blancher. If the blancher feeds a cooling flume it may not be necessary to use a recirculation pump on low volume systems. Gem has utilized a gooseneck decelerator to accomplish the same sealing function on the infeed end. As the product decelerates it is spread out to the width of the blancher belt. On the infeed end, unless an unlimited quantity of water is available, a recirculation pump is required.