Low Delta T ( ∆T) Issues

1- Network Pipe's  Heat Gain Factor :

Since the District Cooling (DC) network covers several buildings , it runs , in many cases , few kilometers buried in the soil. The 2 pipes system(Supply and Return main pipes) line gains heats from the surroundings environment ( from the soil if it is buried , or from air if it is running through tunnels).

Moreover ; it employs large size chambers to accommodate the valves and the take offs for buildings it serves .

Some DCSPs have estimated that the pipe network gains around 2 C , which means that the chilled water supply temp. (CWST) that might arrive to their customers at the far end to be around ( CWST leaving the DCP +2 Deg C)  to as a result of this heat gain .

This heat gain is the Low Delta T (LDT) factor originated from the DCP.

2- The Secondary  Chilled Water Pumps  ( District Cooling Side ) deliver  the chilled water with a flowrate  not necessarily exactly matching  the heat load of Customers , due to its control characteristic . This unmatched flow (if  was higher than the actual heat load requirements ) results in lowering the ∆T.

Both of the above mentioned factors result in :

1- Increase the Chilled Water Flow that is pumped from the District Cooling Plant (DCP) .

2- The above mentioned increase in Chilled Water Flow will be pumped to all Customers Energy Transfer Stations (ETS) . So , the initial impact of the LDT ( being generated due to heat gain in the pipe network ) is increase in the Chilled Water Flow and therefore increasing the power consumption of the DCP .

3- Since The Chilled Water coming from the DCP is arriving to the Customer ETS warmer than the designed CWST , this will be reflected to the Customer side as well .

So , by allowing heat gain to be around 2 C ( 3.6 F) , this means , DCSP assumes that some Customers will get their chilled water  at (40+3.6=43.6F, if the plant generated the chilled water at 40F)) .

this changes in DT generated from DCP side will impose its impacts and generate LDT on Customer side too ; which is being neglected :

- Customers Building HVAC designers may have not initially counted for such LDT , therefore , the HVAC system may not operate  at its optimum performance , and it may incur excessive power consumption similar to the impact that this LDT created on DCP side secondary pumps.

- LDT Charges imposed by district Cooling Service Providers (DCSPs) do not count and compensate customers for this LDT being generated their (DCP) side.

Therefore , it would better that the DCSP estimate the temp of their chilled water (CWST) that is expected to arrive at each Customer Building , so that the HVAC designer of the Buildings consider it rather than imposing a clause in the Service Agreement that the CWST is +l- 3.6 F ( +/- 2C) which may not be counted for by most HVAC designers 

Operational data of few district cooling plants reveals that during peak summer , Delta T (∆T) gets improved in general compared to rest of the year ; the colder the weather , the worst the LDT will be .  this reflects that the  weather has a contribution to Low Delta T (LDT) due to the fact that ;

- The HVAC system is designed for peak summer hours in general .

- The partial load that characterize the load  during off peak.

Occupancy level also plays a role rising the LDT issue ;  the lower the occupancy rate , the lower the partial load rate and therefore the lower the LDT 

Buildings are in general  utilizes types of controls designed for unitary systems :

- FCUs are equipped with few speed levels fans (usually 3 speed levels)  , which means the the fan does not deliver the exact flow required for any particular load other than the full load and  the level equivalent to the delivered flow . therefore the CWRT is not expected to be as the designed .

- FCU heat exchanger : The heat exchanger has its characteristic at partial load and does not necessarily deliver the designed CWRT according to the load , giving rise to LDT .

- The Control Valve (CV) : The CV does not work as expected at the full range of the flow ; it has a range of flow where it is best performed 

- Secondary Chilled Water ( Customer Side) Speed Pump's Speed Control :

Due to the above factors , the Chilled Water Pumps  receives the chilled water whose return Temp ( CWRT) are somewhat lower the design  when it is required to pump more flow than required due to the non operational FCUs  ; therefore , it pumps higher flow and resisting the improvement of LDT.

The HVAC hydronic  of Buildings usually connects several FCUs in a single header . So ; whenever any space being switched off ( being unoccupied , under renovation or maintenance ) it mixes up the outlet chilled water from those connected FCUs which mixes the CWRTs from different FCUs and therefore , the average CWRT is somewhat lower than the design CWRT which lowers the DT . 

The design of district Cooling Plants (DCPs) usually imposes few parameters which are usually commercially driven  and not necessarily reducing the impact of LDT .

The designer have the choice from early days of the design to reduce the LDT impacts .

this trend , itself justifies , that the Customers should not be charged for LDT since the DCSP have alternative solutions from early design stages  .

As it elaborated above , most of the factors that gives rise to LDT are beyond the control of the Customer .The blame that it is  the design or operational are not justified with real assessments .

Customers are advised , by DCSPs to make few retrofits in their HVAC which cost them additional cost ; while the DCP originated factors are not been treated .

Therefore , the LDT fees are imposed by the Service Providers should be assessed case by case to evaluate the Party who is the most driving toward LDT , and what could be done by both Parties rather than advising additional cost on the Customer alone .