Designing Low Delta T- immune HVAC hydronic systems

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1 Designing Low Delta T- immune HVAC hydronic systems Abstract The paper focuses on a common phenomenon in HVAC hydronic systems called "Low Delta T Syndrome". Conventional techniques traditionally used to mitigate the issue are examined, along with their inadequacy. A new, promising type of the control device is examined in greater detail, along with its impact on the hydronic systems design. The paper shows how the new technology provides the long-sought solution to the Low Delta T problem. Factors affecting performance of HVAC hydronic systems A major source of worry to generations of HVAC designers has been the so-called Low Delta T syndrome. All traditionally designed HVAC hydronic systems are susceptible to this degrading condition to some extent. An important gauge of total heating and cooling system performance, Delta T establishes the rate of flow necessary to satisfy a given load. T 24 tons = GPM T = BTUH 500 GPM Figure 1. Delta T calculations In almost every chiller plant in operation, Delta-T falls well short of design levels, particularly at lower load range. This results in higher pump and chiller energy usage. A number of papers have been published on the subject over the years (Kirsner 1996, 1995; Lizardos 1994; Sauer 1989; Fiorino 1996; Avery 1997; Mannion 1988). Some of the common manifestations of the problem are: lost capacity, decrease in facility comfort level, wasted energy, increased emissions and excessive system complexity. Let s examine it in closer detail: Lost Capacity A system with Low Delta T can become flow-limited and not capable of delivering the full design capacity (i.e. a 500 TR chiller w/ 12 deg. T requires 1000 GPM. If the system can only pump 1000 GPM and has 9 deg. T, then the chiller can only deliver 375 Tons.) Decrease in Facility Comfort Level Low Delta T often leads to blending of supply water with return water, which produces higher Chilled Water Supply Temperature (CHWST). This leads to degradation of space temperature and humidity control at the facility served. Wasted Energy The need for additional running equipment (chillers, pumps, tower fans, etc.) to process and distribute more water to meet the load amounts to wasted energy. System instability and poor control can lead to simultaneous heating and cooling, which adds to energy being wasted. 1

2 Increased Emissions Emissions increase due to the need to run extra equipment to meet the load requirements. Excessive System Complexity: Past attempts to address the Low Delta T problem involved fairly dramatic techniques affecting system design, such as: complex reset sequences, custom pumping strategies, building decoupling, adding bypass valves, replacing coils, etc. These techniques, expensive to design and implement, often simply mask the problem without addressing the underlying issue of Low Delta T. To sum up: poor Delta T performance requires additional flow to serve the cooling or heating load. This in turn leads to excess equipment being run, loss of capacity, wasted energy and system instability. The goal for HVAC designer is to address these problems, in order to heat and cool more space with less energy, equipment and system complexity. Common Causes of Low Delta T A multitude of factors can contribute to deteriorating Delta T performance of an actual chiller plant: 1) Broken or Dirty Coils. 2) System Pressure Fluctuations. 3) Improper Valve Sizing. 4) Poor Valve Control. 5) Rising Chilled Water Supply Temperature. 6) Bypasses and Three-Way Valves. Generic solutions fail to address Low Delta T condition Over the years, a number of various techniques have been used in the industry to correct systems exhibiting Low Delta T syndrome, and with a varied degree of success. These are, along with their respective shortcomings: New, higher Delta T design coils: expensive step that can increase system fan static pressure. Reverse return piping: requires more installation time, additional cost and extra pumping energy. Decoupling: requires extra pumping equipment and control devices, and raises CHWST on the coils. Reset of the return water temperature control: raises Exhaust Water Temperature (EWT) at the coils and lowers cooling capacity 2

3 Rebalancing: balancing is not exact science and can be a challenging procedure to do it right. Any time a hydronic system is altered, it needs a rebalancing. Leaving Air Temperature (LAT) reset: achieves higher Delta T by sacrificing comfort level (i.e. room temperature). Loop tuning: does not work well in continuous pressure change environments; the flow still varies in response to pressure changes. Colder CHWST: adds extra operational costs required to produce lower CHWST. The strategies listed above often take a substantial capital expense to implement. Yet there s no guarantee they will address the problem adequately. Valve a central piece of the Control Loop HVAC designers may overlook at times importance of the control valve to the overall system performance. Yet valve is a key part of the system: demand is set by temperature, while the valve controls the flow and the pump sets the head. Hence, choice of a valve can have a profound impact on performance metrics, such as Delta T. Conventional Valve characteristics For decades, HVAC designers have been limited in their choice to pressure-dependent control valves. The ASHRAE Handbook HVAC Systems and Equipment, Chap. 42 (1) discusses a typical valve s reaction to changes in the position. It has a chart showing that a valve performs as advertised only when pressure differential across the valve stays constant, a scenario that seldom occurs. From full load to partial load, pressure differential across control valve can vary significantly. For example, 50% valve position may equal 10% flow at design differential, or it may equal 50% flow at partial load. Figure 2. Conventional valve pressure performance 3

4 Hydronic systems tend to be dynamic with system pressure changing continuously, as valves open and close, and pumps start, stop, and change speed. A change in pressure will lead to change in flow through any device, even if the heating or cooling load remains constant. On an air handler, when the flow changes, so does the heat transfer rate. The thermostat picks up the change and attempts to reset the valve to a proper flow matching the new pressure condition. With conventional control valve, the actuator strokes to move the valve to the right position to provide optimal flow for the load. This can take time and often leads to an inefficient condition called valve hunting. Commercial-quality globe valves are particularly poor at low flow. They are unable to modulate adequately and perform much like the On/Off switches. Conventional valve technology has proven to be inadequate for keeping the flow matched exactly to the load. Their drawbacks lead to dramatically reduced heat transfer, excess flow and low Delta T. P across valve changes with pressure fluctuations Coil flow varies regardless of the load Pressure-Independent Valve characteristics Pressure-Independent Control Valve (PICV) is a relatively recent innovation in hydronic applications. A pressure-independent valve is fundamentally different, in that the pressure drop across the control surface is mechanically regulated. The valve immediately responds to variations in pressure and maintains accurate and steady flow in spite of the pressure fluctuations. Mechanically, the piston and spring in the valve function to maintain a small constant differential pressure across the control surface within the valve. (See Fig.3). The pressure-regulating portion of the valve consumes all the remaining pressure in the circuit. The large spring helps to clear debris and ensure stable flow. Figure 3. Pressure Independent valve performance 4

5 P across the control surface is held constant Coil flow varies only with actuator action PICVs are sized by flow rate alone over a wide range of the system operating pressure. A change in pressure upstream or downstream of the valve will not change the flow rate. To change the flow rate, the stem must be rotated. With pressure-independent valves, flow through heating and cooling coils remains constant despite system pressure variations. Flow can only change when the heating or cooling load changes. Pressure-independent valves immediately compensate for any change in pressure, maintaining internally a constant differential pressure and flow across the controlled surface. This helps achieve optimized coil performance, resulting in better heat transfer and less flow required. The table below summarizes key differences between the two valve types: Conventional Valves Pressure Independent Valves Flow through the coil varies as system pressure changes. Typical valve sizing practice results in poorly sized control valves throughout the system. Use of balancing valves can limit flow and add to the system pressure drop. Flow through the coil remains constant despite system pressure changes. Typically sized by flow only. No balancing valves required, even when the system design changes or expands. Table 1. Conventional valves vs. PICVs PICV-based Design solves Low Delta T problem Having a constant flow, matched precisely to the heating or cooling load allows HVAC designer to optimize heat transfer while minimizing water circulation in the system. That, in return, leads to higher Delta-T values sustained across all load conditions. Thomas Durkin showed in Evolving Design Of Chiller Plants (4) how PICVs, when installed on air handler coils, provide at once the long-sought solution to the low Delta T problem (see Figure 4): 5

6 Figure 4. PICVs in the Variable primary flow configuration with the control valves actually controlling correctly, cooling coil T was now at or above design. Two things immediately evident: the pumps can be accurately and smoothly controlled at a much lower setpoint, leading to energy savings; and low T no longer existed. All previous efforts were about making low T a non-issue, this approach solves it. With PICVs on the cooling coils, the system finally obeys the fundamentals of heat transfer. At all part-load conditions, return water is consistently above design, with no change in room comfort level Durkin concludes: PICVs installed on the air handler coils impact the overall system performance by delivering: Better Control Energy Savings Elimination of Ghost Energy and Low Delta T With system now operating at a higher Delta T, less flow is used to meet load requirements, translating nto less pumping and fewer equipment to run. The benefits are twofold: conserving energy while increasing available Heating and Cooling system capacity. 6

7 PICV Benefits recapped Reduced System Cost Simplified system design and operations Design with fewer pumps and using smaller pipes Reduced maintenance & elimination of balancing Greater system diversity ensured Increase in Available Capacity Serve more space with existing system Extend thermal storage capacity Overcome pump and piping constraints Energy Savings Less flow used for heating and cooling Less equipment to run in the central plant Enables condensing boilers to condense Eliminates or cuts down simultaneous heating & cooling Selecting your PICV Vendor For HVAC designers considering PICV use in their projects, it s important to evaluate vendors against the following checkpoints: Written Delta T Performance Guarantee Valve manufacturer must provide a written guarantee that the heating and cooling coils will meet or exceed Delta T performance at all loads as projected by an ARI-certified program. The manufacturer will reimburse the full purchase price if this criteria is not met. High Flow Accuracy The valve must maintain flow within the minimal tolerance over the full range of operating pressures. High Valve Rangeability A ratio of the maximum controllable flow to the minimum controllable flow, this is a key parameter of the valve performance. Higher rangeability translates into more precise control the valve will deliver, and less energy wasted in heating and cooling. Rangeability of 50 to 1 is considered acceptable, while 100 to 1 is considered ideal. All Flow Rates Operation Allows system to function across a wide range of operating scenarios. Cntd. on next page 7

8 Large Flow Passages in order to minimize the pressure drop at Full flow, it is important for a valve to be designed with large flow passages. Factory Calibration Control valves should be individually flow-tested and factory verified to deviate no more than ±5% over the operating pressure range. A calibrated performance tag listing actually flow rates measured at 10 rotational increments needs to accompany each unit. Ease of Maintenance to ensure maximum uptime and minimal operating costs look for a valve designed with maintenance requirements in mind. DeltaPValves from FlowControl: your key to attaining a higher Delta T Flow Control Industries, Inc. is the industry pioneer in PICV applications for hydronic systems, having introduced DeltaPValves to the HVAC market in Backed by a strong portfolio of patents and design and application expertise, Flow Control Industries, Inc. remains the leader in quality and performance by continuously improving product design through customer feedback and innovation. Highlights that set DeltaPValve products apart are: Written Delta T Performance Guarantee Unique to Flow Control, the Delta T guarantee comes from the extensive experience with variety of different facility types in different climates. It is critical for making your next HVAC project energy efficient. High Flow Accuracy Within +/- 5% over the wide operating pressure range (5-70 psid). High Valve Rangeability Minimum DeltaPValvesrangeability is 100:1. All Flow Rates Up to 4000 gpm with a low pressure drop. Factory set maximum flow rate. Optional remote reset through a building management system. 8

9 Large Flow Passages DeltaPValves feature large passages which limit pressure drop at full flow to 5 psid. Ease of maintenance DeltaPValves are serviceable in-line and designed with ability to perform maintenance or repairs without removing valves from the piping. They feature 3 P/T ports for indication, system troubleshooting, pump control, and commissioning. Conclusion Adopting a pressure independent control approach for a new or existing HVAC hydronic system project is an effective way to address the Low Delta T problem. As an added benefit the system operators will see: optimized coil performance decreased energy consumption increased capacity For more information, visit Flow Control Industries on the Web at Further Reading: 1. The ASHRAE Handbook HVAC Systems and Equipment, 2005 American Society of Heating, Refrigerating and Air-Conditioning Engineers ADJUSTABLE PISTON 2. USPTO Patent No. 4,893,649 CONSTANT FLOW RATE CONTROLLER VALVE 3. USPTO Patent No. 5,301,713 FLOW CONTROL VALVE HAVING 4. Thomas Durkin, Evolving Design Of Chiller Plants ASHRAE Journal, (Vol. 47, No. 11, November 2005) 9