The heart of the hydraulic system: circulation pump
Hello and welcome to the new edition of the HVAC Education Hub newsletter. Today I want to discuss circulation pumps and I want to hear your feedback about them.
One of the key tools for understanding pump performance is the pressure drop/flow rate diagram. The diagram below illustrates the relationship between the pressure a pump can generate and the corresponding flow rate it can achieve.
- The x-axis typically represents the flow rate (often in m3/h or L/min)
- The y-axis shows the pressure drop (usually in kPa or meters of head)
The curve on this diagram represents the pump's performance at different operating points between minimum and maximum speed:
Understanding this diagram is crucial for selecting the right pump for your heat pump system, ensuring it can overcome the system's pressure drop while maintaining the required flow.
Nominal flow rate
In heating systems, the nominal flow rate is a critical parameter. This is the optimal flow rate at which the heat pump achieves its rated capacity and efficiency. For most heat pumps, this is calculated based on a temperature difference (ΔT) of 5K between the flow and return water.
A quick formula to remember:
Nominal flow rate (L/min) ≈ 3 x Heat pump capacity (kW)
For example, a 12 kW heat pump would have a nominal flow rate of about 36 L/min. Maintaining this flow rate is crucial for optimal heat transfer.
NOTE: However, we discussed different ΔT and how this affects efficiency/heat transfer in last week's newsletter: Delta T (ΔT) = 5 °C or maybe this is not fixed.
Fixed speed vs. Modulating pumps
When it comes to circulation pumps, we have two main types: fixed speed and modulating (variable speed) pumps.
Fixed speed pumps:
- Operate at a constant speed regardless of system demand
- Simpler and often less expensive initially
- Can lead to higher energy consumption
Modulating pumps:
- Can adjust their speed based on system demand
- Offer significant energy savings, especially in variable load conditions
- Provide better system control and can extend the life of the heat pump
- Often use PWM (Pulse Width Modulation) for precise control
I know that most of the manufacturers (residential, inverter, up to 16 kW) have modulating pumps but sometimes I hear that some of them still use fixed speed.
In most modern heat pump installations, modulating pumps are preferred. They can match the flow rate to the heat pump's current output, ensuring optimal ΔT and maximizing system efficiency across various operating conditions.
Conclusion and feedback
I think the pump is the most important part of the system and if the software is capable of adapting flow rates and demand in a proper way, the system will perform really well.
FEEDBACK:
For everyone who has, please send me the pump curve and a short explanation (model, nominal capacity/flow rate) of different manufacturers here in a message or by email: hvaceducationhub@gmail.com. I am curious how is control logic done, especially with fixed speed pump. Thanks!
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Thank you for reading and see you next Wednesday! ?
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Retired Mechanical Engineer, Heat Pump Specialist, One Time Refrigeration Engineer and Mechanic
3 个月It's important to remember or be aware that the flow rate affects the condensing of refrigerant. If the flow changes too quickly or by too much the expansion valve and compressor speed can't keep up resulting in a smashed compressor. This is why there's a limit to the ability of the pump modulation to manage the water TD. Open loop systems work best because they avoid sudden changes in flow allowing the gas controls to work at their optimum. It's not all about the water flow.
Managing Director Heacol Consultants
4 个月Mario Dodi? My Delta T pump controller is nearly ready, I should have the first samples in a month or so. It will be interesting to put it on the test bench with various heat pumps and see the difference in performance. It is going to be fun.
HVAC & Building Services Engineer
4 个月I’m one of the owners of a fixed speed flow/head ASHP. Both primary ASHP & secondary pump are fixed speed. The system flow rate on both sides is fixed, at a point on the pump duty curves, so delta T is the variable to modulate capacity. Delta T can drop to 1.6 and the ASHP happily works at low modulation without cycling. The key here is the hydronic balancing of multiple emitters (20) on a sub-optimal extended legacy radiator system. It’s balanced at a fixed flow. It requires reasonably high velocity and head pressure in the secondary pump to do that. The piping circuit isn’t ideal. I’ve tried the secondary pump on a lower setting and the system balancing is then out - the furthest radiators lacking in flow and heat output capacity. A PWM pump would have the same issue. There’s an efficiency/comfort trade off here - higher power consumption of the fixed speed (3) secondary pump, but offset by better comfort control. ‘It works for me’ as they say.
Degree educated ex heating installer
4 个月Both boilers and heat pumps target a variable flow temperature through compensation controls, frequently at temperatures almost as low as room temperature or just above. Surely delta t measurement becomes impossible and irrelevant, yet the boiler can still be firing with a 1c delta t between flow and return, same with heat pumps. So I’m not sure delta t between flow and return I s a critical patameter for controlling a pump (althouugh it is for the movement of heat between two substances )
Managing Director at Rob Berridge Heating Design Consultants
4 个月I know we’ve used this quite a lot but it’s compelling evidence that we need to improve: 15 to 20% of the entire worlds electrical energy wastage is taken up by dead headed (over pumped) and oversized pumps. That’s potentially billions and billions of wasted kWh’s! #WastageCrisis