Some notes in the desgin of the domestic hot water system

19/09/2019

Currently, Sigma is executing many Grade A projects such as Marriott Da Nang, Mikazuki, Flamingo Cat Ba ... with the appearance of the domestic  hot water system. However, many hot water systems in the projects have not been fully evaluated factors, affecting the ability to work of the system, leading to unsatisfactory requirements during use. application (hot water delay time, microorganism growth control, energy optimization ....)

The renovation of the hot water system does not meet the requirements when operating is really a difficult problem, if renovating, it also comes with great costs, so the design attention from the beginning to eliminate irrationalities and make appropriate adjustments is essential. Here are some notes in the
desgin of the domestic hot water system.

I.  Parameters to consider in design

  1. Delay time of hot water at outlet
  2. Heat lost via the surface of hot water pipe
  3. System balancing  and minimum dead legs
  4. Legionella control

II.  Some notes in the desgin of the domestic hot water system

1.  Delay time of hot water

Deylay time of hot water is determine by flow velocity of hot water  and total pipe length of hot water supply pipe that is calcutated form return point to outlet point:

Relationship between the maximum length of the pipe and the pipe diameter and hot water delay time
 
 Delay time (s) 5­­­­a 10a,b 15b 20b,c
Lmax (m) @ D20-PN20 4.6 9.2 13.9 18.5
Lmax (m) @ D25-PN20 2.6 5.2 7.9 10.5
Lmax (m) @ D32-PN20 0.9 1.9 2.8 3.8
 
Notes:
Specifed by flow at outlet point Q=0.2 l/s at out
  1. Delay time of hot water correspond to five-star hotel: 5~10s
  2. Delay time of hot water correspond to four-star hotel: 10~15s
  3. Delay time of hot water correspond to apartment: 15~20s

2.  Hot water return pipe arrangement

Selected way which hot water return pipe connected to hot water supply pipe with tie in point arranged at riser, branch, or last outlet depend on required delay time of hot water, architecture, and invested cost
  • Apartment:

Figure 1
 
  • With the requirement of measuring the flow of hot water used, so the hot water return pipe must connect  branch of hot water supply pipe with tie in point is arranged at apartments and the location of the return point (the tie in point of hot water supply and hot water return pipe line) needs to be done in the most reasonable way to ensure delay time of hot water at all outlet points
  • Hotel room

Figure 2: Return point arranged at riser
  • Special  area required to supply hot water with shortest delay time


Figure 3: Return point arraged at last outlet
 

3.  Calculation of return flow Qh  (m3/s)

Return flow rate has to be designed in dependence of the heat loss of the pipework. 
Formular of hot water return flowQ­b=åL*q/(r*cp*DT) 
Therin:
  • åL: Total length of hot water supply pipe in system calculate from return points to hot water soure
  • q: Heat loss from the pipe line (w)
  • r: Density of water (988kg/m3 at 50oC)  
  • Cp: Specific heat of water (4182 J/kgoC at 50 oC)
  • DT: Temperature drop (oc)

4.  Calculation of pressure drop

  • Preussre drop is calculated for furthest circuit by Hazen-Williams formualar

5. System balancing

The aim to hot water return flow  equalizely in all ciruit the system must to design to ensure hydraulic balancing
  • Balancing pressure in the main riser
Pressure reducing valve must be arranged in both of supply and return pipe and adjusted to ensure static pressure P2=P2’ (Figure A)
 

Figure A
 
  • Flow balancing
Calculation hydraulic balancing is very hard to ensure equalizely pressure lost bettwen circuits. The invention of the balancing valve, especially the automatic balancing valve, greatly reduced the complexity of the hydraulic calculation. Using Balacing valve shown as Figure B to control the maximum flow 

Figure B
  • Thermal balancing 
With a large hot water system, if selected the return option on all circuits, it means that  all circuits being resotred even enough heat or not enough heat. Such an option would result in a great deal of power loss and affect the amount of hot water used, for that reason in order to optimize system operating costs, the choice of a thermostatic valve (thermal balancing valve) to control hot water return to temperature is necessary (only return circuits that do not guarantee the minimum temperature required to maintain the system).

 

6. Required temperature in hot water system

  • The control of legionella require temperature throughout the hot water system that avoids lengionella growth and reduce delay time of hot wate , so the temperature of a centralized hot water tank should not be bolow 60oC and in circulation system must be above 55oC (Germany) or 50oC (UK)

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