convert imperial formula to SI 1

[skyhooks1]

i need some help on understanding a formaula i found in the ASHRAE book, Ground Source Heat Pumps. it relates to the cooling capacity of a ground water system. it is,

gpm = ((qc x 12000)/(500 x (t_gwo-t_gwi))x((EER+3.41)/EER).

the first part of this formula is obviously;
m=q/(Cp(t2-t1). and the EER is the performance of the system. not a problem!

what i can't understand is where the constant 3.41 has come from and should i include it in my formula, can anyone help?

 

[briand2]

skyhooks1,

I work in SI units myself (mostly)!

If you multiply the Coefficient of Performance (CoP) of the refrigeration system by 3.41, you get the EER (Energy Efficiency Rating). The reason the figure 3.41 appears in the formula you quoted is that the 3.41 has to be taken out again for that particular calculation.

Now for the reasoning behind the 3.41:

As you know, the CoP of the system is the ratio of useful cooling output to power input. In SI units,

CoP = kWcooling / kWinput (which is a unitless ratio).

In areas using imperial units, cooling is measured in British Thermal Units per hour (Btuh) (or, more usually, thousands of Btuh, because Btuh is a small unit, being just over a quarter of a Watt). However, they still measure electrical power in SI units (W, or kW). So, their "user friendly" CoP (EER), being cooling power divided by input power, gets expressed in hybrid units (Btuh/W, or 000's Btuh/kW).

Now, for a simple conversion, 1 W is equivalent to about 3.416 Btuh. This is where the 3.4 comes from.

Sorry if this rambled a bit, but hopefully you'll be able to follow it anyway.

Brian

 

[skyhooks1]

yeah, that is great,

thankyou very much.