Secondary Refrigerant Pipe Flow Rates

Transcription

1 Secondary Refrigerant Pipe Res 1.0 Introduction The purpose of this technical bulletin is to provide allowable velocities and flow res for a range of various fluids for a range of pipe sizes. The velocities and flow res given in Table 1 are based on an allowable pressure drop of 410 Pa/m which gives relively small pipe sizes without making much difference to the pump size in our sort of applicions. The velocity has been limited to 2.8 maximum. 2.0 Equions The calculions are based on the equions given in 1997 ASHRAE Fundamentals Handbook 2.8 which gives the friction factor f based on Reynolds number Re for turbulent flow as f = for Re < 10 5 Re 0.25 and f = for 10 5 < Re < 3 x 10 6 Re Where flow is likely to be laminar Re is calculed from 64 f = for Re < 3000 Re ρdv Where Re = µ Where ρ = density kg/m 3 d = pipe diameter m v = velocity µ = viscosity Ns/m 2 Table 2 give some properties for the fluids listed in Table 1.

2 2.0 Selection Guidance 2.1 Fluid Temperure Difference ( T) Single Cooler The secondary fluid T should be kept as wide as is economic to minimise pipe size. For a single cooler, this will typically mean a minimum of 3K for most chill applicions but 4K is more normal and 5K is a good starting point for applicions where glycol is chilling wer or chilling air above 10ºC Central Systems On a central system, with several rooms different temperures, the guidelines for a single cooler can be used to select flow and return temperures for the coldest room. For a given flow temperure, the return temperures from the warmer coolers will be relively higher giving economic designs. For example, if we consider a typical supermarket regional distribution centre with different rooms say 0ºC, +3ºC and +15ºC. For the lowest temperure room 0ºC, if we select a glycol flow temperure of say -8ºC then this defines the glycol flow temperure for all coolers. The return temperure would be 3K above this or -5ºC. For the room with a design air temperure of +3ºC, the economic glycol return temperure is around -2ºC. For the room with a design air temperure of +15ºC room, the economic glycol return temperure is around +8ºC. The mixed return temperure would be around -2ºC. Although the above selection basis increases the size of some of the air coolers, the selection is economic because of the corresponding reduction in pipe and valve sizes. It also increases the secondary refrigerant return temperure to the evaporor increasing the evaporor temperure difference and reducing its size. Other advantages include smaller pipes for the defrosting and heing circuit and reduced pump size and power Oil Cooler Circuits Where we are cooling oil using a secondary fluid and removing the he in a separe coil in the evaporive condenser we should design on a T of 10K (or 12K if the oil cooler has a large enough surface).

3 2.2 Defrost and Heing Res The size of the warm pipes for our typical four pipe systems for glycol cooled air coolers can be estimed on a flow of 1 / 3 of the cold flow for both defrosting and heing. De Issue Description Originor 02/10/01 1 DJH/AMG 20/06/08 B 30% ethylene glycol 40ºC added to tables 1 and 2. Minor text corrections. SM

4 Pipe Size NB Wer 1 Tyfoxit 1.15 Hycool 20 30% Ethylene 4 20% Ethylene 35% Ethylene 20% Propylene 35% Propylene TB 126 Table 1 Allowable ocities and Res

5 Wer 1 Tyfoxit 1.15 Hycool 20 30% Ethylene 4 20% Ethylene 35% Ethylene 20% Propylene 35% Propylene Specific He cp kj/kgk Density ρ kg/m Viscosity µ Ns/m TB126 Table 2 Fluid Properties