Fluoropolymer cores are becoming the material of choice for sanitary applications. PTFE, PFA, and FEP are three common fluoropolymers, with a typical range of -65°F–450°F (-53°C–230˚C).
Fluoropolymer cores are the most chemically inert ones available. They are non-aging, non-stick, easy to clean, and can withstand repetitive steam cleaning. Like metal, they also have a low absorption rate. In addition, today's fluoropolymer cores boast flexibility comparable to that of silicone. However, fluoropolymers are highly permeable.
With many fluoropolymer hoses, you can specify a carbon-black-filled core if your fluid requires static dissipation. Static sparking can damage hose and pose a safety hazard.
Thermoplastic (nylon) hoses can contain high pressures and often are chosen for hydraulic applications. They are available in sizes up to one inch and have a typical range of -40°F–200°F (-40°C–93°C).
|Convoluted Cores -- Figure 3. Helical design (top) has a single
spiral while annular design (bottom) uses a series of
Rubber hoses provide a temperature range comparable to thermoplastic ones. They only can handle low pressure but can be crushed without permanent damage. So, such hoses provide an economical option for general-purpose duties. While the other types of hoses typically are available only in diameters up to 2 inches, rubber hoses also come in larger sizes.
Besides tube material, you must consider wall construction — whether the hose should have a smooth-bore core or convoluted core that allows it to bend like a flexible straw. Your application's requirement for hose bendability as well as pressure, flow and drainability needs will guide your decision.
Choose smooth bore (Figure 2) if precise flow control is a priority — it lacks wall irregularities that can cause interruptions. Smooth bore also promotes drainability. Its primary disadvantage is kinking, particularly in larger diameters. Reinforcement layers, discussed later, can help prevent kinking. All core materials except for metal are offered in smooth bore.
Convoluted cores are best when flexibility is the priority. The walls of the tube are folded in a pattern that increases the hose's ability to bend without kinking. Metal and fluoropolymer cores are offered with convolutions.
Convoluted cores come in two types — helical and annular. The helical design, found primarily in fluoropolymer cores, is a single convolution that spirals down the length of the hose (Figure 3 top). The annular design, typical in metal cores, consists of a series of connected rings (Figure 3 bottom).
Helical convolutions promote better flow downstream than annular ones; so, choose a helical design when flow maintenance and drainability are important. Annular metal cores come with deep convolutions and so provide maximum flexibility. However, convoluted metal hoses don't suit operations with repetitive movements because such movements can cause metal fatigue and breakage.