1-19 - Lecture 20 Emitter Selection & Design I....

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Lecture 20 Emitter Selection & Design I. Introduction • • • • • • • • There are hundreds of models, sizes and types of emitters, sprayers, bubblers, and others, available from dozens of manufacturers Prices of emitters can change frequently Some emitters have longer life than others, but cost more Some emitters have better pressure compensating features, but cost more Some emitters have better flushing capabilities, but cost more It is very difficult to know which is the “correct” emitter for a particular design, and usually there are a number of emitters that could work and would be acceptable for a given system Thus, the selection of an emitter involves knowledge of the different types, their prices, their availability, and their performance Experience on the designer’s part is valuable, and emitter selection will often involve a process of elimination II. Long-Path Emitters • • • So-called “spaghetti” tubing is a typical example of a long-path emitter Long-path emitters also come in spiral configurations (Fig. 20.1 of the textbook) These can be represented by an equation used for capillary flow under laminar conditions: c g π D4 H = ν qK (372) where lc is the length of the flow path; D is the inside diameter; H is the pressure head; ν is the kinematic viscosity (a function of water temperature); q is the flow rate; K is for units conversion; and g is the ratio of force to mass • • • • • The above equation is only approximately correct for long-path emitters The above equation is based on circular cross-sections, which is typical The above equation assumes laminar flow, which may not be the case Note that the flow rate is proportional to the fourth power of the diameter, so the diameter is a very important dimension Note also that the flow rate is inversely proportional to the length (double the length and get half the flow rate) Sprinkle & Trickle Irrigation Lectures Page 225 Merkley & Allen • When is it valid to assume laminar flow? Consider that a Reynolds number of 4,000 is probably as high as you can go without transitioning from laminar to turbulent flow: VD 4Q = < 4,000 ν π νD (373) or, Q < 15D @ 10°C, with Q in lph and D in mm cool short warm hot long longest • • In black PE lateral hose, sunlight warms the water significantly as the velocity slows down, and water viscosity decreases Long-path emitters would ideally be progressively longer along the lateral to compensate and provide a more uniform discharge along the lateral III. Tortuous- and Short-Path Emitters • • • • Tortuous-path emitters also have long paths, but not laminar flow. This is because the path has many sharp bends, and is in the form of a maze Tortuous-path emitters tend to behave hydraulically like orifices, and so do many short-path emitters Flow rate is nearly independent of the viscosity, at least over typical ranges in viscosity Many short-path emitters have pressure compensating features Merkley & Allen Page 226 Sprinkle & Trickle Irrigation Lectures IV. Orifice Emitters • • • Many drip emitters and sprayers behave as orifices The orifice(s) are designed to dissipate energy and reduce the flow rate to an acceptable value Flow rate is approximately proportional to the square root of the pressure V. Line Source Tubing • • • Single-chamber tubing provides less uniformity than dual-chamber tubing In dual-chamber tubing, much of the head loss occurs through the orifices between the two chambers. The outer chamber is somewhat analogous to a manifold or header. The flow rate equation for dual-chamber tubing can be expressed as: q = a 'K 2 2g Hno ( 2 1 + no ) (374) where a’ is the area of the outer orifice; K is an empirical coefficient; H is the pressure head; and no is the number of outer orifices per inner orifice (no > 1.0) • See Fig. 20.2 in the textbook VI. Vortex and Sprayer Emitters • • • Vortex emitters have a whirlpool effect in which the water must exit through the center of the whirlpool Energy is dissipated by the friction from spinning in a chamber, and from exiting through an orifice in the center As mentioned in a previous lecture, the exponent on the pressure head is approximately equal to 0.4 (in the discharge equation). Thus, these can usually be considered to be (partially) pressure compensating VII.Pressure Compensating Emitters • • • Pressure compensating emitters usually have some flexible or moving parts These types of emitters tend to need replacement or repair more often than most of the simpler emitter designs, therefore incurring higher maintenance cost Figure 20.3 of the textbook shows one design approach for a pressure compensating emitter Sprinkle & Trickle Irrigation Lectures Page 227 Merkley & Allen • As defined previously, pressure compensating emitters always have a pressure head exponent of less than 0.5 (otherwise they aren’t considered to be pressure compensating) VIII.Self-Flushing Emitters • • • • • In this category there are continuous and periodic flushing emitters Periodic flushing emitters perform their self cleaning when the lateral is filled (before it reaches full operating pressure), and when the lateral is emptied. In other words, they typically flush once per day. Continuous-flushing emitters have flexible parts that can stretch to allow solid particles to pass through Fig. 20.4 in the textbook shows an example of one such design These can be sensitive to temperature changes and are not normally pressure compensating IX. Calculating the Discharge Exponent • • • • You can calculate the exponent, x, based on a pair of measured flow rates and pressure heads Recall a rule of logarithms: log (ax) = x (log a) The solution can be obtained graphically, but is more quickly accomplished with calculators and electronic spreadsheets If you have more than two pairs of q and H, then you can take the logarithmic transformation of the equation and perform linear regression; however, the regression will be mathematically biased toward the smaller values Design Approach & Example I. Review of Example Designs • We will review example designs in Chapter 21 of the textbook, and discuss design alternatives and parameters affecting efficiency, etc II. Summarized Trickle Irrigation Design Process • • These are 15 basic steps, following the material presented in Chapters 17-24 of the textbook, that can be followed for the design of many trickle systems These are basic steps and represent a summary of the generalized design process, but remember that each design situation will have some unique features 1. Collect data on the crop, climate, soil, topography, and irrigation water quality, field shape & size, water availability. Merkley & Allen Page 228 Sprinkle & Trickle Irrigation Lectures 2. Select an emitter and determine an emission point layout such that 33% < Pw < 67%. This will determine the number of emitters per plant, Np. Emitter selection may involve field testing to determine the wetted width (or diameter), w. 3. Calculate dx, fx, and Td. Note that fx will almost always be greater than 1.0. 4. Select a target value for EU (usually 70-95%; see Table 20.3) and estimate the peak-use transmission ratio, Tr (usually 1.00-1.10; see Table 19.3). 5. Calculate the leaching requirement, LRt, based on crop type and irrigation water quality. 6. Let f = 1 day (usually), then dn = Td. Calculate the gross application depth, d. 7. Calculate the gross volume of water required per plant per day, G. ⎛ dS pS r ⎞ G = K⎜ ⎟ f ⎝ ⎠ (375) 8. Calculate the daily hours of operation, Ta, (per station, or subunit) during the peak-use period. Ta = G N pq a (376) 9. Determine the number of operation stations based on Ta (with more stations, the system capacity is lower). If Ta = 24 hrs, then Ns = 1 If Ta = 12 hrs, then Ns = 1 or 2 If Ta = 8 hrs, then Ns = 2 or 3, and so on 10. Adjust Np and qa so that TaNs is equal to, or slightly less than, 90%(24 hrs/day) = 21.6 hrs/day. First, try adjusting qa because this is usually less expensive than increasing Np. If the emitter is pressure compensating, or if qa must be greatly altered, you may need to change Np (or you may need to select a different emitter). 11. Having determined the value of qa, calculate the minimum allowable emitter discharge, qn Sprinkle & Trickle Irrigation Lectures Page 229 Merkley & Allen qn = q a EU 100 (1.0 − 1.27 νs ) (377) Note that if EU is high and νs is high, it could be that qn > qa (but this would not be a reasonable calculation result!) 12. Calculate the average (nominal) and minimum lateral pressure heads 1/ x ⎛q⎞ h=⎜ ⎟ ⎝ Kd ⎠ (378) 1/ x ⎛q ⎞ hn = ha ⎜ n ⎟ ⎝ qa ⎠ (379) 13. Calculate the allowable change in pressure head in an operating station ∆Hs = 2.5 (ha − hn ) (380) 14. Calculate Qs, Vs, and Ot. 15. Finally, size the laterals, headers, manifolds and mainline(s) according to hydraulic design criteria. Merkley & Allen Page 230 Sprinkle & Trickle Irrigation Lectures ...
View Full Document

This note was uploaded on 03/01/2012 for the course BIE 6110 taught by Professor Sprinkle during the Fall '03 term at Utah State University.

Ask a homework question - tutors are online