Pumps Don’t Add Head: Valves Do (Part One)
By Greg Cunniff, Applications Engineering Manager, Taco, Inc.
Net-zero green buildings are in vogue today and for a very good reason: they are meant to operate at no net energy consumption, drawing no net energy from the electrical grid. This sustainable capability is accomplished through a combination of energy efficiency and on-site energy generation, often through co-generation.
A reader recently asked us if there is data to support a claim that electrical consumption in a system employing Taco’s LOFlo injection mixing system would be less than a traditional design with control valves. To be more precise, what he is getting at is if there would be less energy used in a system configuration using single pipe with two sets of pumps in a primary-secondary configuration vs. a system using one larger pump set along with control valves in the building’s zones.
To answer the question, we need to examine how we calculate pump energy consumption, which is a simple first law problem. And we should take into account today’s variable speed drives which are specifically designed to reduce energy consumption.
First off, it doesn’t make any difference how the pump horsepower is split up. What counts is the total flow and the total head in a system – e.g., placing 40% of the pump head on one set of pumps and 60% with the other. The total pump horsepower will be the same if the efficiencies of the pumps are the same.
In Taco’s case, using our single pipe LoadMatch® and LOFlo® systems, we can achieve savings in pump horsepower by splitting the pump horsepower between primary and terminal secondary pumps. This allows for a pump controlled system without control valves and a self balancing system without balance valves. This will save between 15 and 20 ft. of pump head on the total system, depending on how the valves are sized.
For our LOFlo injection mixing system, a three pump system with primary, secondary injection and secondary terminal unit pumps, it doesn’t make any difference if we have two secondary pumps instead of one. Adding pumps does not add head – it eliminates head.
Here is an example of how one can calculate the head in a system with and without using LOFlo.
To calculate pump horsepower multiply head x flow x a conversion factor, divided by the pump efficiency in both loops/ The calculation of total pump horsepower would therefore be as follows:
Horsepower = 200 gpm x 68 ft./3960/.70 (pump efficiency)
= 4.9 hp
Horsepower = 200 gpm x 46 ft. (head of primary and secondary house loop)/3960/.70 (primary pump efficiency) + 200 gpm x 5 ft. (head of terminal unit loop)/ 3960/.25 (efficiency of LoadMatch circulators)
= 3.2 + 1.0
= 4.2 hp
This represents a savings in pump horsepower of 14%, which is not trivial. In the real world achieving net zero is actually impractical from a cost standpoint as well as impossible from a first law standpoint. However, we can use it to our advantage since it is our firm contention that hydronic systems are more efficient than air systems.
Next up: Comparing Total System Horsepower: VAV vs. Chilled Beam-LOFlo Systems
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