I need to provide an estimation of the BHP of a fan used in a HVAC system (water-source heat-pump). I have only limited information, the fan nameplate motor is 1 HP, it will run at 750 cfm is rated at 0.5 in w.c. of ESP. I don't have any information about its efficiency (motor or total). What could be a good estimation for its BHP?

Since most motor reach their peak efficiency around 75% load I was thinking about doing 1 HP * 0.75 / eff and using 85% as its efficiency. Would this be a bad estimation?

  • $\begingroup$ What is the BHP of a fan? $\endgroup$ – Karlo Aug 1 '16 at 12:02
  • $\begingroup$ I would define fan BHP as the horsepower needed to drive the fan under specified operation conditions. @DecemberUser, do you have another definition? I don't see how the efficiency of the motor is useful in estimating anything other than the input power to the motor. I don't think that it is useful to assume the motor has been selected to operate at 75% of the fan requirement. The question is rather confusing, but if a fan BHP estimate is the objective, a fan efficiency estimate is required. If the fan BHP is more than 1, a motor overload is likely. I don't think anyone can say much more. $\endgroup$ – Charles Cowie Aug 1 '16 at 22:01
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    $\begingroup$ If you have the fan nameate data you can contact the fan manufacturer and request a fan curve. 1 hp is relatively small, so the difference between BHP and the motor power may be significant (relative to one another). You would want total static pressure so you can see where the fan is operating on the fan curve. But there is no valid way to interpret BHP from can motor nameplate data. Plus the fan motors power factor may not be what you expect either. Sorry this doesn't offer a good Methodolgy, but I wanted to make it clear that there is no equation to get BHP from fan motor HP. $\endgroup$ – Prevost Sep 24 '16 at 2:13
  • $\begingroup$ @CharlesCowie - your definition of BHP is incorrect. For a fan, it is the amount of power required to stop the airflow (so the amount of power of the air, which you can find by using the correct units and multiplying fan airflow by the pressure difference across the fan, which the fan curve already does for you). And fan efficiency doesn't come into play either. You are right in saying the motor efficiency is irrelevant and the assumption of the motor operating point at 75%. Both do not directly relate and are therefore not directly useful for solving the question (with accuracy). $\endgroup$ – Prevost Sep 24 '16 at 2:21
  • $\begingroup$ @Prevost The definition that I gave is the BHP of the motor required to drive a fan. That is the usual BHP definition used by fan manufacturers and people involved in selecting motors to drive fans. You could define a BHP for the air output of the fan, but that is not really useful. The question didn't really make much sense to begin with, but all that really matters is what the OP meant by BHP, and we will probably never know that. $\endgroup$ – Charles Cowie Sep 24 '16 at 5:00

One way to do it would be to first estimate the internal static pressure ($ISP$) of your system (due to coils etc...) and add it to the maximum external static pressure ($ESP$) to get the total static pressure ($TSP$, in in. w.c). Then, since you know the airflow ($Q$, in cfm) I would calculate the ideal fan power ($P_{fan}$, in W) required as follow:

$P_{fan}= Q * TSP * \frac{249}{2118}$

Then you can calculate the brake horsepower ($BHP$, in HP) by using the fan efficiency ($μ$, in %), if you don't have it, use 60% (to stay conservative).


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