Back to basics. Setting HEVs for Liquid Overfed Evaporators. 13 th Annual IRC R&T Forum May 8-9, 2013

Transcription

1 Back to basics Setting HEVs for Liquid Overfed Evaporators 13 th Annual IRC R&T Forum May 8-9, 2013 Todd B. Jekel, Ph.D., P.E. Assistant Director, IRC

2 Overview Valve characteristics Nomenclature Setting hand expansion valves Recommendations Other methods 2

3 Valve Characteristics Refrigerating Specialties Globe Hand Exp. Hansen Technologies Hansen Technologies 3

4 Identifying Hand Exp. Valves Bodies are similar or identical Hand-expansion valves have bar stock handles or yellow stem caps Globe Hand Exp. Hansen Technologies 4

5 Locating Hand-expansion Valves Liquid Overfed Evaporator (isolation valves not shown) hot gas pumped liquid wet (protected) suction 5

6 Recirculation or Overfeed? Recirculation rate, N R, is the total liquid mass flow delivered to the evaporator divided by the mass flow of liquid evaporated Overfeed rate, N O, is the unevaporated liquid mass flow divided by the evaporated mass flow 6

7 Recirculation or Overfeed? The are related, but they are not synonymous! N R = N O + 1 Pump sizing requires N R Evaporator manufacturers often specify N O KNOW THE DIFFERENCE! 7

8 Determining volume flow rate From mass flow rate [lb m /min] or tons GPM m NR tons , h h o i Circulating Rate, N R Recirculated Liquid Flow Rate (gpm/ton) Liquid Saturation Temperature [ F] / ( C) -60 (-51) -40 (-40) -20 (-29) 0 (-18) +20 (-7) +40 (+4) 2: : : :

9 Flow Coefficient, C v A simplified equation often used for determining flow rate through a valve given pressure drop, flow coefficient, and fluid specific gravity is: GPM = C v p S where GPM is the volume flow rate in gallons/min, Dp is the pressure drop in psi, S is the specific gravity, and C v has units of gpm/psi 0.5 9

10 Hansen Technologies, Bulletin G510b Setting 1. Determine the evaporator capacity 2. Determine the valve man. & size 3. Determine the target C v 4. Estimate turns open from table or graph 10

11 Setting, continued R/S Bulletin 82-00B 11

12 Example Evaporator 15 tons operating at +20 F, manufacturer recommends 3:1 overfeed Valve 1 straight pattern, welded, assume 10 psi pressure difference Results Hansen Technologies: 3.7 turns open R/S: 1.8 turns open 12

13 Pressure difference The pressure difference across the hand expansion valve is needed to determine the flow rate We don t usually have pressure gauges on both sides of the valve We can estimate the pressure difference 13

14 Estimating the pressure diff. Gather the following: Pump pressure difference Elevation difference between the pump and the hand exp. valve Subtract 5 psi for the evaporator s button orifices Vessel Net Static Can get more sophisticated, but setting the valve isn t too sensitive 14

15 Ammonia Static Head Temperature [ F] ft per psi psi per ft

16 Pressure Difference Example Pumped +20 F liquid Pump differential pressure is 35 psi Elevation difference is 50 ft between the pump and the hand exp. valve Results in a 14 psi drop Assume 5 psi across the orifices Pressure Diff. Estimate 16 psi 16

17 Updated Example Same evaporator from earlier, now with 15 psi instead of the assumed 10 psi Hansen Technologies: 3.4 turns open R/S: 1.6 turns open Effected the setting by <0.3 turns 17

18 Setting Recommendations Ensure you are using the an estimate of the actual temperature difference (TD) on the evaporator Many plants have evaporator capacity at 10 F TD, but operate at higher TDs Obtain an actual supply liquid pressure reading near the evaporator if possible 18

19 Setting Recommendations Recommend setting for slightly lower overfeed than manufacturer s recommendation for cataloged capacity (i.e. clean) for frosted evaporators Evaporator capacity will drops by to 30-40% as it frosts Important if you are using a VFD on the evaporator fan 19

20 Other methods Frost Method: close fully, then slowly reopen until u-bends and suction riser begins to frost (opening the hand valve further may cause a loss of evaporator capacity) Experimental Method: Measure the air temperature difference across the evaporator at different hand expansion valve settings, maximize the temperature difference 20

21 QUESTIONS? 21