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Unformatted text preview: Hydronic systems
A brief introduction Hydronic System Defined A hydronic system carries heat between one or more conditioning units, and one or more loads using a liquid liquid. The most common liquid for hydronic systems is water with a small percentage of additives. Motivation Water can carry approximately 2000 times as much heat per unit volume The air volume required for ventilation is much less than that required for p (maybe ) temperature control ( y 1/4) Conditioning units are often concentrated g y in a single mechanical room for multistory structures, while outside air ducts can just run from the nearest outside wall. Constant Flow Distribution Constant Flow 3-way valve either sends water to the coil 3 way or to the bypass Each control valve always handles the same amount of flow P id constant fl Provides t t flow t th chillers, to the hill which was important for early models Once the system is balanced it the pressures never change Constant Flow Drawbacks Pumping energy is always at its maximum value At low loads water returns to the chillers at nearly the supply temperature Primary/Secondary Primary Secondary Decoupler should be large diameter and short to minimize pressure drop Allows distribution and production flows to be different If production and di t ib ti fl d ti d distribution flows match, t h all water to distribution supply is from chillers hill Distribution can be variable speed Primary Secondary Drawbacks If distribution flow is more than production flow, then some return water is mixed with supply water to terminals If production flow is higher than distribution flow, flow then some conditioned water is wasted Variable Primary Flow Variable Primary Flow Least pump energy of the various schemes at part load Return and supply water are never mixed mixed, improving efficiency Variable Primary Flow Drawbacks Old chillers (with simple controls) cannot cope with variable flows and could potentially freeze Variable speed pumps have a minimum speed Cooling Coils Typical coil behavior
Heat Transfer vs Flow
60000 40 35 50000 30 40000 BTU/HR Delta T 30000 25 B BTU/HR 20 15 20000 10 10000 5 0 0 5 10 GPM 15 20 25 0 Deg F Typical Design Condition
Heat Transfer vs Flow
60000 40 35 50000 40000 90% of theoretical
BTU/HR 30 25 B T U /H R 30000 20 15 20000 10 10000 5 0 0 5 10 GPM 15 20 25 0 D eg F Delta T Why did you just show that? The most common argument against hydronic balancing is that the control valves will balance themselves themselves. This shows that the control valve cannot detect an overflow since it is typically overflow, controlled by the room temperature. Fl Flow Design I D i Inc. and our sister company d i t TA make balancing valves. Warning
Commercial Content Follows Hydronic Balancing Any fluid network with many parallel branches will have some circuits of less total resistance than others The fluid being pumped through the network will be distributed mostly to the circuits with the least resistance It is seldom th t thi i th d i d i ld that this is the desired condition in a hydronic network Hydronic Balancing A series of controlled resistances are inserted in the network, each with the ability to measure the flow rate The resistances are adjusted to achieve the correct flow at all terminals at the "design condition" (highest expected demand throughout the system) Automatic Balancing The resistances inserted into the network are dynamic, and designed to prevent excess flow to any terminal The resistances adjust to changes in the system pressure as needed Differential Pressure Balancing Differential pressure regulators are inserted into the system at strategic points These regulators prevent overflow and also assist the function of the control valves Some regulators are built into a control valve, some are separate and work with l t d k ith several circuits Unbalanced Consequences Some circuits are starved as others pointlessly waste capacity Efficiency is compromised or ruined The installed conditioning capacity cannot be t b transmitted to th terminals, making itt d t the t i l ki part of the investment pointless ...
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- Winter '07