What is the difference between ASHRAE fitting SR4-2 and ER4-2?

[Kapp]

Both look identical to me, but the pressure loss coefficients are different. I'm trying to understand why the pressure drop would be different if the air flow was going to a fan or coming from a fan. It's not a converging/diverging difference, because that is accounted for in the A0/A1 value. ER4-2 is under negative pressure, but the lighter density should be accounted for in the velocity pressure. I've attached images of the fittings and the loss coefficient table. Any thoughts?

 

[MintJulep]

I'd guess the difference in velocity profiles at fan discharge vs fan suction.

 

[Kapp]

Thanks Mint, I thought that as well, but ASHRAE has an entire section of fan discharge/suction fittings. Which implies that these fittings can be anywhere in the system.